cfq_get_queue: fix possible NULL pointer access

cfq_get_queue()->cfq_find_alloc_queue() can fail, check the returned value.
Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>

Note that this isn't a bug at the moment, since the regular IO path
does not call this path without __GFP_WAIT set. However, it could be a
future bug, so I've applied it.
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>

block/cfq-iosched.c

@@ -1443,8 +1443,11 @@ cfq_get_queue(struct cfq_data *cfqd, int is_sync, struct task_struct *tsk,
 		cfqq = *async_cfqq;
 	}
 
-	if (!cfqq)
+	if (!cfqq) {
 		cfqq = cfq_find_alloc_queue(cfqd, is_sync, tsk, gfp_mask);
+		if (!cfqq)
+			return NULL;
+	}
 
 	/*
 	 * pin the queue now that it's allocated, scheduler exit will prune it