LOCKD: Clear ln->nsm_clnt only when ln->nsm_users is zero

The current code is clearing it in all cases _except_ when zero.

Reported-by: Stanislav Kinsbursky <skinsbursky@parallels.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>
Cc: stable@vger.kernel.org