pNFS: Always set NFS_LAYOUT_RETURN_REQUESTED with lo->plh_return_iomode

When setting the layout return mode, we must always also set the
NFS_LAYOUT_RETURN_REQUESTED flag to ensure that we send a layoutreturn.
Otherwise pnfs_error_mark_layout_for_return() could set the mode, but
fail to send the layoutreturn because another is already in flight.

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