iproute2: cake: make gso/gro splitting configurable

iproute2: cake: make gso/gro splitting configurable

This patch makes sch_cake's gso/gro splitting configurable
from userspace.

To disable breaking apart superpackets in sch_cake:

tc qdisc replace dev whatever root cake no-split-gso

to enable:

tc qdisc replace dev whatever root cake split-gso

Signed-off-by: Toke Høiland-Jørgensen <toke@toke.dk>
Signed-off-by: Dave Taht <dave.taht@gmail.com>
[pulled from netdev list - no API/ABI change]
Signed-off-by: Kevin Darbyshire-Bryant <ldir@darbyshire-bryant.me.uk>