ipc/sem.c: optimize if semops fail

Reduce the amount of scanning of the list of pending semaphore operations:
If try_atomic_semop failed, then no changes were applied.  Thus no need to
restart.

Additionally, this patch correct an incorrect comment: It's possible to
wait for arbitrary semaphore values (do a dec by <x>, wait-for-zero, inc
by <x> in one atomic operation)

Both changes are from Nick Piggin, the patch is the result of a different
split of the individual changes.

Signed-off-by: Manfred Spraul <manfred@colorfullife.com>
Cc: Nick Piggin <npiggin@suse.de>
Cc: Pierre Peiffer <peifferp@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

ipc/sem.c

@@ -445,13 +445,13 @@ again:
 		 * of the completed operation:
 		 * - if the operation modified the array, then restart from the
 		 *   head of the queue and check for threads that might be
-		 *   waiting for semaphore values to become 0.
+		 *   waiting for the new semaphore values.
 		 * - if the operation didn't modify the array, then just
 		 *   continue.
 		 */
 		alter = q->alter;
 		wake_up_sem_queue(q, error);
-		if (alter)
+		if (alter && !error)
 			goto again;
 	}
 }