eventfd: clean compile when CONFIG_EVENTFD=n

Fix gcc warning and add parameter checking when CONFIG_EVENTFD=n:

fs/aio.c: In function 'aio_complete':
fs/aio.c:955: warning: statement with no effect

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Cc: Davide Libenzi <davidel@xmailserver.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>