Btrfs: new subvolume oops fix

Signed-off-by: Chris Mason <chris.mason@oracle.com>

fs/btrfs/super.c

diff --git a/fs/btrfs/super.c b/fs/btrfs/super.c
index eba239cc..6940a90 100644
--- a/fs/btrfs/super.c
+++ b/fs/btrfs/super.c
@@ -2013,6 +2013,8 @@
 	BUG_ON(!trans);
 
 	subvol = btrfs_alloc_free_block(trans, root);
+	if (subvol == NULL)
+		return -ENOSPC;
 	leaf = btrfs_buffer_leaf(subvol);
 	btrfs_set_header_nritems(&leaf->header, 0);
 	btrfs_set_header_level(&leaf->header, 0);
@@ -2022,8 +2024,6 @@
 	memcpy(leaf->header.fsid, root->fs_info->disk_super->fsid,
 	       sizeof(leaf->header.fsid));
 	mark_buffer_dirty(subvol);
-	brelse(subvol);
-	subvol = NULL;
 
 	inode_item = &root_item.inode;
 	memset(inode_item, 0, sizeof(*inode_item));
@@ -2035,6 +2035,8 @@
 
 	btrfs_set_root_blocknr(&root_item, bh_blocknr(subvol));
 	btrfs_set_root_refs(&root_item, 1);
+	brelse(subvol);
+	subvol = NULL;
 
 	ret = btrfs_find_free_objectid(trans, root->fs_info->tree_root,
 				       0, &objectid);