nfsd4: nfsd_shutdown_net needs state lock

A comment claims the caller should take it, but that's not being done.
Note we don't want it around the cancel_delayed_work_sync since that may
wait on work which holds the client lock.

Reported-by: Benny Halevy <bhalevy@primarydata.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>