tipc: enqueue arrived buffers in socket in separate function

The code for enqueuing arriving buffers in the function tipc_sk_rcv()
contains long code lines and currently goes to two indentation levels.
As a cosmetic preparaton for the next commits, we break it out into
a separate function.

Reviewed-by: Ying Xue <ying.xue@windriver.com>
Signed-off-by: Jon Maloy <jon.maloy@ericsson.com>
Signed-off-by: David S. Miller <davem@davemloft.net>