net: Add a software TSO helper API

Although the implementation probably needs a lot of work, this initial API
allows to implement software TSO in mvneta and mv643xx_eth drivers in a not
so intrusive way.

Signed-off-by: Ezequiel Garcia <ezequiel.garcia@free-electrons.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --cc include/net/tso.h
Simple merge

diff --cc net/core/Makefile
Simple merge

diff --cc net/core/tso.c
Simple merge