netfilter: move br_nf_check_hbh_len to utils

Rename br_nf_check_hbh_len() to nf_ip6_check_hbh_len() and move it
to netfilter utils, so that it can be used by other modules, like
ovs and tc.

Signed-off-by: Xin Long <lucien.xin@gmail.com>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Reviewed-by: Nikolay Aleksandrov <razor@blackwall.org>
Reviewed-by: Aaron Conole <aconole@redhat.com>
Signed-off-by: Florian Westphal <fw@strlen.de>