tipc: fix compatibility bug

In commit d999297c
("tipc: reduce locking scope during packet reception") we introduced
a new function tipc_link_proto_rcv(). This function contains a bug,
so that it sometimes by error sends out a non-zero link priority value
in created protocol messages.

The bug may lead to an extra link reset at initial link establising
with older nodes. This will never happen more than once, whereafter
the link will work as intended.

We fix this bug in this commit.

Signed-off-by: Jon Maloy <jon.maloy@ericsson.com>
Signed-off-by: David S. Miller <davem@davemloft.net>