selftests: centralise maybe-unused definition in kselftest.h

selftests: centralise maybe-unused definition in kselftest.h

Several selftests subdirectories duplicated the define __maybe_unused,
leading to redundant code.  Move to kselftest.h header and remove other
definitions.

This addresses the duplication noted in the proc-pid-vm warning fix

Link: https://lkml.kernel.org/r/20250821101159.2238-1-reddybalavignesh9979@gmail.com
Signed-off-by: Bala-Vignesh-Reddy <reddybalavignesh9979@gmail.com>
Suggested-by: Andrew Morton <akpm@linux-foundation.org>
Link:https://lore.kernel.org/lkml/20250820143954.33d95635e504e94df01930d0@linux-foundation.org/
Reviewed-by: Wei Yang <richard.weiyang@gmail.com>
Acked-by: SeongJae Park <sj@kernel.org>
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Acked-by: Mickal Salan <mic@digikod.net>	[landlock]
Cc: Shuah Khan <shuah@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>