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

Cc: James Bottomley <James.Bottomley@HansenPartnership.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>