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

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 <jlayton@kernel.org>
Tested-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: NeilBrown <neilb@suse.de>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>