pNFS: Clear out all layout segments if the server unsets lrp->res.lrs_present

If the server fails to set lrp->res.lrs_present in the LAYOUTRETURN reply,
then that means it believes the client holds no more layout state for that
file, and that the layout stateid is now invalid.

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