Btrfs: fix bh leak on __btrfs_open_devices path

'bh' is forgot to release if no error is detected

Signed-off-by: Xiao Guangrong <xiaoguangrong@cn.fujitsu.com>
Signed-off-by: Chris Mason <chris.mason@oracle.com>
diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
index deca1a0..290100f 100644
--- a/fs/btrfs/volumes.c
+++ b/fs/btrfs/volumes.c
@@ -597,6 +597,7 @@
 			list_add(&device->dev_alloc_list,
 				 &fs_devices->alloc_list);
 		}
+		brelse(bh);
 		continue;
 
 error_brelse: