bpf: Remove ipv6_bpf_stub usage in test_run

bpf: Remove ipv6_bpf_stub usage in test_run

bpf_prog_test_run_skb() uses net->ipv6.ip6_null_entry for
BPF_PROG_TYPE_LWT_XMIT test runs.

It currently checks ipv6_bpf_stub before using ip6_null_entry.
ipv6_bpf_stub will be removed by the CONFIG_IPV6=m support removal
series posted at [1], so switch this check to ipv6_mod_enabled()
instead.

This change depends on that series [1]. Without it, CONFIG_IPV6=m is
still possible, and net->ipv6.ip6_null_entry remains NULL until
the IPv6 module is loaded.

[1] https://lore.kernel.org/netdev/20260320185649.5411-1-fmancera@suse.de/

Cc: Jakub Kicinski <kuba@kernel.org>
Cc: Fernando Fernandez Mancera <fmancera@suse.de>
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>
Reviewed-by: Sun Jian <sun.jian.kdev@gmail.com>
Reviewed-by: Fernando Fernandez Mancera <fmancera@suse.de>
Link: https://lore.kernel.org/r/20260323225250.1623542-1-martin.lau@linux.dev
Signed-off-by: Alexei Starovoitov <ast@kernel.org>