nfsd: Ensure we check stateid validity in the seqid operation checks

After taking the stateid st_mutex, we want to know that the stateid
still represents valid state before performing any non-idempotent
actions.

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