Btrfs: reset path lock state to zero

We forgot to reset the path lock state to zero after we unlock the path block,
and this can lead to the ASSERT checker in tree unlock API.

Reported-by: Slava Barinov <rayslava@gmail.com>
Signed-off-by: Liu Bo <bo.li.liu@oracle.com>
Signed-off-by: Josef Bacik <jbacik@fusionio.com>