selftests: drv-net: gro: make large packet math more precise

selftests: drv-net: gro: make large packet math more precise

When constructing the packets for large_* test cases we use
a static value for packet count and MSS. It works okay for
ipv4 vs ipv6 but the gap between ipv4 and ip6ip6 is going to
be quite significant.

Make the defines calculate the worst case values, those
are only used for sizing stack arrays. Create helpers for
calculating precise values based on the exact test case.

Reviewed-by: Willem de Bruijn <willemb@google.com>
Link: https://patch.msgid.link/20260402210000.1512696-7-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>