If the callback daemon is signalled, but is unable to exit because it still
has users, then we need to flush signals. If not, then svc_recv() can
never sleep, and so we hang.
If we flush signals, then we also have to be prepared to resend them when
we want the thread to exit.
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>