selftests/net: use destination options instead of hop-by-hop

The GRO self-test, gro.c, currently constructs IPv6 packets containing a
Hop-by-Hop Options header (IPPROTO_HOPOPTS) to ensure the GRO path
correctly handles IPv6 extension headers.

However, network elements may be configured to drop packets with the
Hop-by-Hop Options header (HBH). This causes the self-test to fail
in environments where such network elements are present.

To improve the robustness and reliability of this test in diverse
network environments, switch from using IPPROTO_HOPOPTS to
IPPROTO_DSTOPTS (Destination Options).

The Destination Options header is less likely to be dropped by
intermediate routers and still serves the core purpose of the test:
validating GRO's handling of an IPv6 extension header. This change
ensures the test can execute successfully without being incorrectly
failed by network policies outside the kernel's control.

Fixes: 7d1575014a63 ("selftests/net: GRO coalesce test")
Reviewed-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: Anubhav Singh <anubhavsinggh@google.com>
Link: https://patch.msgid.link/20251030060436.1556664-1-anubhavsinggh@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>