nfsd: Migrate the stateid reference into nfs4_lookup_stateid()

Allow nfs4_lookup_stateid to take the stateid reference, instead
of having all the callers do so.

Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>