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

commit bc2df47a408f2d64cf81bcfd0f6e3e14c84cb0ab upstream.

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: Benny Halevy <bhalevy@tonian.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>