tipc: Optimize handling excess content on incoming messages

Remove code that trimmed excess trailing info from incoming messages
arriving over an Ethernet interface.  TIPC now ignores the extra info
while the message is being processed by the node, and only trims it off
if the message is retransmitted to another node.  (This latter step is
done to ensure the extra info doesn't cause the sk_buff to exceed the
outgoing interface's MTU limit.) The outgoing buffer is guaranteed to
be linear.

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