nfsd4: return expired on unfound stateid's

nfsd4: return expired on unfound stateid's

Commit 9dc6951ef600a7b1a81485d1fae109c0db6d0578 "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>