ubsan: Fix pointer overflow error message

In __ubsan_handle_pointer_overflow(), fix the condition for determining
whether a pointer operation overflowed or underflowed. Currently, the
function reports "underflowed" when it should be reporting "overflowed"
and vice versa.

Example of incorrect error reporting:
void *foo = (void *)__UINTPTR_MAX__;
foo += 1;

UBSAN:
pointer operation underflowed ffffffff to 00000000

Fixes: 4e3fb2f ("ubsan: add clang 5.0 support")
Signed-off-by: Michal Orzel <[email protected]>
Acked-by: Andrew Cooper <[email protected]>

xen/common/ubsan/ubsan.c

@@ -513,7 +513,7 @@ void __ubsan_handle_pointer_overflow(struct pointer_overflow_data *data,
 	ubsan_prologue(&data->location, &flags);
 
 	pr_err("pointer operation %s %p to %p\n",
-	       base > result ? "underflowed" : "overflowed",
+	       base > result ? "overflowed" : "underflowed",
 	       _p(base), _p(result));
 
 	ubsan_epilogue(&flags);