Btrfs: update to use fs_state bit

Now that we use bit operation to check fs_state, update
btrfs_free_fs_root()'s checker, otherwise we get back to
memory leak case.

Signed-off-by: Liu Bo <bo.li.liu@oracle.com>
Signed-off-by: Josef Bacik <jbacik@fusionio.com>
Signed-off-by: Chris Mason <chris.mason@fusionio.com>