bpf: remove ipv6_bpf_stub completely and use direct function calls

bpf: remove ipv6_bpf_stub completely and use direct function calls

As IPv6 is built-in only, the ipv6_bpf_stub can be removed completely.

Convert all ipv6_bpf_stub usage to direct function calls instead. The
fallback functions introduced previously will prevent linkage errors
when CONFIG_IPV6 is disabled.

Signed-off-by: Fernando Fernandez Mancera <fmancera@suse.de>
Tested-by: Ricardo B. Marlière <rbm@suse.com>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Reviewed-by: Martin KaFai Lau <martin.lau@kernel.org>
Link: https://patch.msgid.link/20260325120928.15848-10-fmancera@suse.de
Signed-off-by: Jakub Kicinski <kuba@kernel.org>