kexec: Annotate struct crash_mem with __counted_by

Prepare for the coming implementation by GCC and Clang of the __counted_by
attribute. Flexible array members annotated with __counted_by can have
their accesses bounds-checked at run-time checking via CONFIG_UBSAN_BOUNDS
(for array indexing) and CONFIG_FORTIFY_SOURCE (for strcpy/memcpy-family
functions).

As found with Coccinelle[1], add __counted_by for struct crash_mem.

[1] https://github.com/kees/kernel-tools/blob/trunk/coccinelle/examples/counted_by.cocci

Cc: Eric Biederman <ebiederm@xmission.com>
Cc: kexec@lists.infradead.org
Acked-by: Baoquan He <bhe@redhat.com>
Link: https://lore.kernel.org/r/20230922175224.work.712-kees@kernel.org
Signed-off-by: Kees Cook <keescook@chromium.org>