Btrfs: btrfs_ioctl_default_subvol: Revert back to toplevel subvolume when arg is 0

This patch makes it possible to set BTRFS_FS_TREE_OBJECTID as the default
subvolume by passing a subvolume id of 0.

Signed-off-by: chandan <chandan@linux.vnet.ibm.com>
Reviewed-by: David Sterba <dsterba@suse.cz>
Signed-off-by: Josef Bacik <jbacik@fusionio.com>
Signed-off-by: Chris Mason <chris.mason@fusionio.com>

fs/btrfs/ioctl.c

diff --git a/fs/btrfs/ioctl.c b/fs/btrfs/ioctl.c
index fd6557e..fba259a 100644
--- a/fs/btrfs/ioctl.c
+++ b/fs/btrfs/ioctl.c
@@ -3310,7 +3310,7 @@
 	}
 
 	if (!objectid)
-		objectid = root->root_key.objectid;
+		objectid = BTRFS_FS_TREE_OBJECTID;
 
 	location.objectid = objectid;
 	location.type = BTRFS_ROOT_ITEM_KEY;