Initialize variables to fix -Werror=maybe-uninitialized in gnttab.c

[Copilot]

Build fails on drivers/xen/gnttab.c when compiling with -Werror due to uninitialized variable warnings for ret and gfn in gnttab_get_pages().

Changes

• Initialize ret = 0 and gfn = 0 at declaration in gnttab_get_pages()

The compiler cannot statically prove these variables are assigned before use in the error logging path, though runtime logic guarantees they are (loop always executes at least once when npages > 0).

void *gnttab_get_pages(unsigned int npages)
{
-	int ret;
+	int ret = 0;
	void *page_addr;
	unsigned int removed;
-	xen_pfn_t gfn;
+	xen_pfn_t gfn = 0;

Original prompt

Building C object zephyr/CMakeFiles/zephyr.dir/drivers/xen/events.c.obj
[37/133] Building C object zephyr/CMakeFiles/zephyr.dir/drivers/xen/gnttab.c.obj
FAILED: zephyr/CMakeFiles/zephyr.dir/drivers/xen/gnttab.c.obj
ccache /opt/toolchains/zephyr-sdk-0.17.4/aarch64-zephyr-elf/bin/aarch64-zephyr-elf-gcc -DKERNEL -DK_HEAP_MEM_POOL_SIZE=16384 -DPICOLIBC_DOUBLE_PRINTF_SCANF -DZVFS_OPEN_SIZE=0 -D_POSIX_THREAD_SAFE_FUNCTIONS=200809L -D__LINUX_ERRNO_EXTENSIONS__ -D__ZEPHYR__=1 -I/__w/zephyr/zephyr/kernel/include -I/__w/zephyr/zephyr/arch/arm64/include -I/__w/zephyr/zephyr/twister-out/xenvm_xenvm_gicv3/zephyr/tests/arch/common/stack_unwind/arch.common.stack_unwind.arm64/zephyr/include/generated/zephyr -I/__w/zephyr/zephyr/include -I/__w/zephyr/zephyr/twister-out/xenvm_xenvm_gicv3/zephyr/tests/arch/common/stack_unwind/arch.common.stack_unwind.arm64/zephyr/include/generated -I/__w/zephyr/zephyr/soc/xen -I/__w/zephyr/zephyr/lib/libc/picolibc/include -I/__w/zephyr/zephyr/lib/posix/c_lib_ext/getopt -I/__w/zephyr/zephyr/lib/midi2/. -I/__w/zephyr/zephyr/subsys/testsuite/include -I/__w/zephyr/modules/hal/microchip/include -I/__w/zephyr/modules/hal/st/sensor/inc -I/__w/zephyr/modules/hal/ti/mspm0/source/ti/devices/msp/. -I/__w/zephyr/modules/hal/ti/mspm0/source/ti/devices/msp/m0p -I/__w/zephyr/modules/hal/ti/mspm0/source/ti/devices/msp/peripherals -I/__w/zephyr/modules/hal/ti/mspm0/source/ti/devices/msp/peripherals/m0p -I/__w/zephyr/modules/hal/ti/mspm0/source/ti/devices/msp/peripherals/m0p/sysctl -isystem /__w/zephyr/zephyr/lib/libc/common/include -fno-strict-aliasing -Werror -Og -imacros /__w/zephyr/zephyr/twister-out/xenvm_xenvm_gicv3/zephyr/tests/arch/common/stack_unwind/arch.common.stack_unwind.arm64/zephyr/include/generated/zephyr/autoconf.h -fno-common -g -gdwarf-4 -fdiagnostics-color=always -mcpu=cortex-a72 -mabi=lp64 --sysroot=/opt/toolchains/zephyr-sdk-0.17.4/aarch64-zephyr-elf/aarch64-zephyr-elf -imacros /__w/zephyr/zephyr/include/zephyr/toolchain/zephyr_stdint.h -Wall -Wformat -Wformat-security -Wno-format-zero-length -Wdouble-promotion -Wno-pointer-sign -Wpointer-arith -Wexpansion-to-defined -Wno-unused-but-set-variable -Werror=implicit-int -fno-pic -fno-pie -fno-asynchronous-unwind-tables -ftls-model=local-exec -fno-omit-frame-pointer -fno-reorder-functions --param=min-pagesize=0 -fno-defer-pop -fmacro-prefix-map=/__w/zephyr/zephyr/tests/arch/common/stack_unwind=CMAKE_SOURCE_DIR -fmacro-prefix-map=/__w/zephyr/zephyr=ZEPHYR_BASE -fmacro-prefix-map=/__w/zephyr=WEST_TOPDIR -ffunction-sections -fdata-sections -mno-omit-leaf-frame-pointer -moverride=tune=no_ldp_stp_qregs -specs=picolibc.specs -std=c17 -MD -MT zephyr/CMakeFiles/zephyr.dir/drivers/xen/gnttab.c.obj -MF zephyr/CMakeFiles/zephyr.dir/drivers/xen/gnttab.c.obj.d -o zephyr/CMakeFiles/zephyr.dir/drivers/xen/gnttab.c.obj -c /__w/zephyr/zephyr/drivers/xen/gnttab.c
In file included from /__w/zephyr/zephyr/include/zephyr/logging/log.h:11,
from /__w/zephyr/zephyr/drivers/xen/gnttab.c:31:
/__w/zephyr/zephyr/include/zephyr/logging/log_core.h: In function 'gnttab_get_pages':
/__w/zephyr/zephyr/include/zephyr/logging/log_core.h:221:9: error: 'gfn' may be used uninitialized [-Werror=maybe-uninitialized]
221 | z_log_minimal_printk("%c: " fmt "\n",
| ^~~~~~~~~~~~~~~~~~~~
/__w/zephyr/zephyr/drivers/xen/gnttab.c:202:19: note: 'gfn' was declared here
202 | xen_pfn_t gfn;
| ^~~
/__w/zephyr/zephyr/include/zephyr/logging/log_core.h:221:9: error: 'ret' may be used uninitialized [-Werror=maybe-uninitialized]
221 | z_log_minimal_printk("%c: " fmt "\n",
| ^~~~~~~~~~~~~~~~~~~~
/__w/zephyr/zephyr/drivers/xen/gnttab.c:199:13: note: 'ret' was declared here
199 | int ret;
| ^~~
cc1: all warnings being treated as errors
ninja: build stopped: subcommand failed.

INFO - /__w/zephyr/zephyr/twister-out/xenvm_xenvm_gicv3/zephyr/tests/arch/common/stack_unwind/

fix this bug

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