selftests/net: fix gro.c compilation failure due to non-existent opt_ipproto_off

Linux 6.6 does not have an opt_ipproto_off variable in gro.c at all (it
was added in later kernel versions), so attempting to initialize one
breaks the build.

Fixes: c80d53c4 ("selftests/net: fix uninitialized variables")
Cc: <stable@vger.kernel.org> # 6.6
Reported-by: Ignat Korchagin <ignat@cloudflare.com>
Closes: https://lore.kernel.org/all/8B1717DB-8C4A-47EE-B28C-170B630C4639@cloudflare.com/#t


Signed-off-by: John Hubbard <jhubbard@nvidia.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>