nfsd4: return expired on unfound stateid's

Commit 78155ed7

 "nfsd4: distinguish
expired from stale stateids" attempted to distinguish expired and stale
stateid's using time information that may not have been completely
reliable, so I reverted it.

That was throwing out the baby with the bathwater; we still do want to
return expired, but let's do that using the simpler approach of just
assuming any stateid is expired if it looks like it was given out by the
current server instance, but we can't find it any more.

This may help clients that are recovering from network partitions.

Reported-by: Bian Naimeng <biannm@cn.fujitsu.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>