lockd: hold a reference to nlmsvc_serv while stopping the thread.

Both nfsd and nfsv4-callback take a temporary reference to the svc_serv
while calling svc_set_num_threads() to stop the last thread. lockd does
not.

This extra reference prevents the scv_serv from being freed when the
last thread drops its reference count. This is not currently needed
for lockd as the svc_serv is not accessed after the last thread is told
to exit.

However a future patch will require svc_exit_thread() to access the
svc_serv after the svc_put() so it will need the code that calls
svc_set_num_threads() to keep a reference and keep the svc_serv active.

So copy the pattern from nfsd and nfsv4-cb to lockd, and take a
reference around svc_set_num_threads(.., 0)

Reviewed-by: Jeff Layton <[e-mail]>
Tested-by: Jeff Layton <[e-mail]>
Signed-off-by: NeilBrown <[e-mail]>
Signed-off-by: Chuck Lever <[e-mail]>

fs/lockd/svc.c

···
 	unregister_inet6addr_notifier(&lockd_inet6addr_notifier);
 #endif
 
+	svc_get(nlmsvc_serv);
 	svc_set_num_threads(nlmsvc_serv, NULL, 0);
+	svc_put(nlmsvc_serv);
 	timer_delete_sync(&nlmsvc_retry);
 	nlmsvc_serv = NULL;
 	dprintk("lockd_down: service destroyed\n");