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 <[email protected]>
Signed-off-by: Dave Taht <[email protected]>
[pulled from netdev list - no API/ABI change]
Signed-off-by: Kevin Darbyshire-Bryant <[email protected]>
(cherry picked from commit 3e8a9389961cd866b867740a2f71c2a0af97ab56)