lockd: close potential race with rapid lockd_up/lockd_down cycle

If lockd_down is called very rapidly after lockd_up returns, then
there is a slim chance that lockd() will never be called. kthread()
will return before calling the function, so we'll end up never
actually calling the cleanup functions for the thread.

Signed-off-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>