netfilter: nf_conntrack_bridge: add support for IPv6

br_defrag() and br_fragment() indirections are added in case that IPv6
support comes as a module, to avoid pulling innecessary dependencies in.

The new fraglist iterator and fragment transformer APIs are used to
implement the refragmentation code.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: David S. Miller <davem@davemloft.net>