_pnfs_return_layout had the following problems:

- it did not call pnfs_free_lseg_list on all paths
- it unintentionally did a forgetful return when there was no outstanding io
- it raced with concurrent LAYOUTGETS
Signed-off-by: NFred Isaman <iisaman@netapp.com>
Signed-off-by: NTrond Myklebust <Trond.Myklebust@netapp.com>