pNFS: Clear NFS_LAYOUT_RETURN_REQUESTED when invalidating the layout stateid

We must ensure that we don't schedule a layoutreturn if the layout stateid
has been marked as invalid.

Fixes: 2a59a041

 ("pNFS: Fix pnfs_set_layout_stateid() to clear...")
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>
Cc: stable@vger.kernel.org # v4.8+