netfilter: ipv6: nf_defrag: avoid/free clone operations

commit 6aafeef0
("netfilter: push reasm skb through instead of original frag skbs")
changed ipv6 defrag to not use the original skbs anymore.

So rather than keeping the original skbs around just to discard them
afterwards just use the original skbs directly for the fraglist of
the newly assembled skb and remove the extra clone/free operations.

The skb that completes the fragment queue is morphed into a the
reassembled one instead, just like ipv4 defrag.

openvswitch doesn't need any additional skb_morph magic anymore to deal
with this situation so just remove that.

A followup patch can then also remove the NF_HOOK (re)invocation in
the ipv6 netfilter defrag hook.

Cc: Joe Stringer <joestringer@nicira.com>
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>