tipc: same receive code path for connection protocol and data messages

As a preparation to eliminate port_lock we need to bring reception
of connection protocol messages under proper protection of bh_lock_sock
or socket owner.

We fix this by letting those messages follow the same code path as
incoming data messages.

As a side effect of this change, the last reference to the function
net_route_msg() disappears, and we can eliminate that function.

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