tipc: refactor function tipc_sk_recvmsg()

We try to make this function more readable by improving variable names
and comments, plus some minor changes to the logics.

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