tipc: fix retransmission on unicast links

A scenario has been observed where a 'bc_init' message for a link is not
retransmitted if it fails to be received by the peer. This leads to the
peer never establishing the link fully and it discarding all other data
received on the link. In this scenario the message is lost in transit to
the peer.

The issue is traced to the 'nxt_retr' field of the skb not being
initialised for links that aren't a bc_sndlink. This leads to the
comparison in tipc_link_advance_transmq() that gates whether to attempt
retransmission of a message performing in an undesirable way.
Depending on the relative value of 'jiffies', this comparison:
    time_before(jiffies, TIPC_SKB_CB(skb)->nxt_retr)
may return true or false given that 'nxt_retr' remains at the
uninitialised value of 0 for non bc_sndlinks.

This is most noticeable shortly after boot when jiffies is initialised
to a high value (to flush out rollover bugs) and we compare a jiffies of,
say, 4294940189 to zero. In that case time_before returns 'true' leading
to the skb not being retransmitted.

The fix is to ensure that all skbs have a valid 'nxt_retr' time set for
them and this is achieved by refactoring the setting of this value into
a central function.
With this fix, transmission losses of 'bc_init' messages do not stall
the link establishment forever because the 'bc_init' message is
retransmitted and the link eventually establishes correctly.

Fixes: 382f598f ("tipc: reduce duplicate packets for unicast traffic")
Acked-by: Jon Maloy <jmaloy@redhat.com>
Signed-off-by: Hamish Martin <hamish.martin@alliedtelesis.co.nz>
Signed-off-by: David S. Miller <davem@davemloft.net>