nfsd: extra checks when freeing delegation stateids

We've had some reports of problems in the refcounting for delegation
stateids that we've yet to track down. Add some extra checks to ensure
that we've removed the object from various lists before freeing it.

Link: https://bugzilla.redhat.com/show_bug.cgi?id=2127067


Signed-off-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>