nfsd4: BUG_ON(!is_spin_locked()) no good on UP kernels

Most frequent symptom was a BUG triggering in expire_client, with the
server locking up shortly thereafter.

Introduced by 508dc6e1 "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>