Changes

testsuites/gtplab-sysmo2017/tunend: Disable GRO/LRO

This can cause problems on incoming decaps traffic from CN (eg. TCP),
which may end up being joined into a bigger packet which will then fail
to be encapsulated into UDP fitting an MTU of 1500.
This didn't show up specifically during tests since we are crafting UE UDP
traffic with proper size in order to fit the MTU, but it would have
shown up if TCP was used.

Change-Id: Iccc6e90e50e73bcadf0ea9f0470850a141a4c389