tipc: allow to build NACK message in link timeout function

Commit 02288248 ("tipc: eliminate gap indicator from ACK messages")
eliminated sending of the 'gap' indicator in regular ACK messages and
only allowed to build NACK message with enabled probe/probe_reply.
However, necessary correction for building NACK message was missed
in tipc_link_timeout() function. This leads to significant delay and
link reset (due to retransmission failure) in lossy environment.

This commit fixes it by setting the 'probe' flag to 'true' when
the receive deferred queue is not empty. As a result, NACK message
will be built to send back to another peer.

Fixes: 02288248

 ("tipc: eliminate gap indicator from ACK messages")
Acked-by: Jon Maloy <jmaloy@redhat.com>
Signed-off-by: Tung Nguyen <tung.q.nguyen@dektech.com.au>
Signed-off-by: David S. Miller <davem@davemloft.net>