tipc: refactor function tipc_sk_anc_data_recv()

We refactor tipc_sk_anc_data_recv() to make it slightly more
comprehensible, but also to facilitate application of some additions
to the code in a future commit.

Reviewed-by: Xin Long <lucien.xin@gmail.com>
Tested-by: Hoang Le <hoang.h.le@dektech.com.au>
Signed-off-by: Jon Maloy <jmaloy@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>