Replace function instances of __attribute__((unused)) with
__maybe_unused to suppress warnings.

Cc: David Howells <dhowells@redhat.com>
Signed-off-by: David Rientjes <rientjes@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>