tipc: Minor optimizations to received message processing

This patch enhances TIPC's handler for incoming messages in two
ways:
- the trivial, single-use routine for processing non-sequenced
  messages has been merged into the main handler
- the interface that received a message is now identified without
  having to access and/or modify the associated sk_buff

Signed-off-by: Allan Stephens <allan.stephens@windriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>