nfsd: fix year-2038 nfs4 state problem

Someone with a weird time_t happened to notice this, it shouldn't really
manifest till 2038.  It may not be our ownly year-2038 problem.

Reported-by: Aaron Pace <Aaron.Pace@alcatel-lucent.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>