Merge branch 'selftests-drv-net-gro-more-test-cases'

Jakub Kicinski says:

====================
selftests: drv-net: gro: more test cases

Add a few more test cases for GRO.

First 4 patches are unchanged from v1.

Patches 5 and 6 are new. Willem pointed out that the defines are
duplicated and all these imprecise defines have been annoying me
for a while so I decided to clean them up.

With the defines cleaned up and now more precise patch 7 (was 5)
no longer has to play any games with the MTU for ip6ip6.

The last patch now sends 3 segments as requested.
====================

Link: https://patch.msgid.link/20260402210000.1512696-1-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>