blk-mq: export blk_mq_freeze_queue()

Commit b4c6a028774b exported the start and unfreeze, but we need
the regular blk_mq_freeze_queue() for the loop conversion.

Signed-off-by: Jens Axboe <axboe@fb.com>
diff --git a/block/blk-mq.c b/block/blk-mq.c
index 1a41d7a..a7d4a98 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -136,6 +136,7 @@
 	blk_mq_freeze_queue_start(q);
 	blk_mq_freeze_queue_wait(q);
 }
+EXPORT_SYMBOL_GPL(blk_mq_freeze_queue);
 
 void blk_mq_unfreeze_queue(struct request_queue *q)
 {