tipc: Fix node lock problems during broadcast message reception

Modifies TIPC's incoming broadcast packet handler to ensure that the
node lock associated with the sender of the packet is held whenever
node-related data structure fields are accessed. The routine is also
restructured with a single exit point, making it easier to ensure
the node lock is properly released and the incoming packet is properly
disposed of.

Signed-off-by: Allan Stephens <allan.stephens@windriver.com>
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>