netfilter: move inline nf_ip6_ext_hdr() function to a more appropriate header.

There is an inline function in ip6_tables.h which is not specific to
ip6tables and is used elswhere in netfilter.  Move it into
netfilter_ipv6.h and update the callers.

Signed-off-by: Jeremy Sowden <jeremy@azazel.net>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>