sunrpc/svc: use store_release_wake_up()

svc_thread_init_status() contains an open-coded
store_release_wake_up().  It is cleaner to use that function directly
rather than needing to remember the barrier.

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