fix: kernel tracing works without CONFIG_CONSTRUCTORS (module notifier) (1.7.4)

[Jo5ta]

Fixes a real portability gap in kernel tracing (1.7.4).

The kernel module registered a traced module's tracebuffers/tracepoints only from a C constructor injected into that module. The kernel calls module constructors only when CONFIG_CONSTRUCTORS is enabled — and it's off in most production kernels (it's pulled in by gcov/KASAN builds). On those kernels the module loads cleanly but the constructor never runs, so nothing registers and every tracepoint fails with invalid in_file_offset(0). Tracing silently does nothing. Our QEMU test only passed because it explicitly enabled CONFIG_CONSTRUCTORS.

Fix: the clltk kernel module now also registers a module notifier (register_module_notifier) and does the registration on MODULE_STATE_COMING / de-registration on MODULE_STATE_GOING. Notifiers run on any kernel and need no cooperation from the traced module. The constructor path is kept for CONFIG_CONSTRUCTORS kernels; registration/de-registration are idempotent, so the two paths agree.

Verified in QEMU on a kernel built with CONFIG_CONSTRUCTORS disabled (confirmed absent in the config): 5 trace files created, zero invalid in_file_offset errors — i.e. the notifier alone registers everything. The module compile/link check (the CI kernel job) also passes with the notifier.

Note: the constructor + notifier both-running case (a CONFIG_CONSTRUCTORS kernel) is idempotent by construction; the constructor code itself is unchanged from what already passed the QEMU test.