btrfs: pass fs_info to btrfs_del_root instead of tree_root

btrfs_del_roots always uses the tree_root.  Let's pass fs_info instead.

Signed-off-by: Jeff Mahoney <jeffm@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>