i386 mmzone: use __maybe_unused

Replace automatic variable instances of __attribute__ ((unused)) with
__maybe_unused.

Cc: Andy Whitcroft <apw@shadowen.org>
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>