Fix silly typo ("smb" vs "smp")

Introduced by commit 6003a93e7bf6c02f33c02976ff364785d4273295

ipc/sem.c

···
 			/* hands-off: q will disappear immediately after
 			 * writing q->status.
 			 */
-			smb_wmb();
+			smp_wmb();
 			q->status = error;
 			q = n;
 		} else {