When block layer fails to map iov, it calls bio_unmap_user to undo

mapping.  Which is good if pages were mapped - but if they were provided
by someone else and just copied then bad things happen - pages are
released once here, and once by caller, leading to user triggerable BUG
at include/linux/mm.h:246.
Signed-off-by: Petr Vandrovec <petr@vandrovec.name>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>

block/blk-map.c

@@ -224,7 +224,7 @@ int blk_rq_map_user_iov(struct request_queue *q, struct request *rq,
 		 */
 		bio_get(bio);
 		bio_endio(bio, 0);
-		bio_unmap_user(bio);
+		__blk_rq_unmap_user(bio);
 		return -EINVAL;
 	}