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>

include/linux/eventfd.h

@@ -19,7 +19,8 @@ int eventfd_signal(struct file *file, int n);
 #else /* CONFIG_EVENTFD */
 
 #define eventfd_fget(fd) ERR_PTR(-ENOSYS)
-#define eventfd_signal(f, n) 0
+static inline int eventfd_signal(struct file *file, int n)
+{ return 0; }
 
 #endif /* CONFIG_EVENTFD */