Most frequent symptom was a BUG triggering in expire_client, with the
server locking up shortly thereafter.
Introduced by 508dc6e110c6dbdc0bbe84298ccfe22de7538486 "nfsd41:
free_session/free_client must be called under the client_lock".
Cc: stable@kernel.org
Cc: Benny Halevy <bhalevy@tonian.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>