tipc: rename media/msg related definitions

The TIPC_MEDIA_ADDR_SIZE and TIPC_MEDIA_ADDR_OFFSET names
are misleading, as they actually define the size and offset of
the whole media info field and not the address part. This patch
does not have any functional changes.

Signed-off-by: Erik Hugne <erik.hugne@ericsson.com>
Signed-off-by: David S. Miller <davem@davemloft.net>