pNFS: Fix bugs in _pnfs_return_layout

We need to honour the NFS_LAYOUT_RETURN_REQUESTED bit regardless of
whether or not there are layout segments pending.
Furthermore, we should ensure that we leave the plh_return_segs list
empty.

This patch fixes a memory leak of the layout segments on plh_return_segs.

Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>