tipc: RDM/DGRAM transport uses new fragmenting and sending functions

We merge the code for sending port name and port identity addressed
messages into the corresponding send functions in socket.c, and start
using the new fragmenting and transmit functions we just have introduced.

This saves a call level and quite a few code lines, as well as making
this part of the code easier to follow. As a consequence, the functions
tipc_send2name() and tipc_send2port() in port.c can be removed.

For practical reasons, we break out the code for sending multicast messages
from tipc_sendmsg() and move it into a separate function, tipc_sendmcast(),
but we do not yet convert it into using the new build/send functions.

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>