bpf: Allow sk_lookup with IPv6 module

This is a more complete fix than d71019b5

 ("net: core: Fix build
with CONFIG_IPV6=m"), so that IPv6 sockets may be looked up if the IPv6
module is loaded (not just if it's compiled in).

Signed-off-by: Joe Stringer <joe@wand.net.nz>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>