net: extract nf_ct_handle_fragments to nf_conntrack_ovs

Now handle_fragments() in OVS and TC have the similar code, and
this patch removes the duplicate code by moving the function
to nf_conntrack_ovs.

Note that skb_clear_hash(skb) or skb->ignore_df = 1 should be
done only when defrag returns 0, as it does in other places
in kernel.

Signed-off-by: Xin Long <lucien.xin@gmail.com>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Reviewed-by: Aaron Conole <aconole@redhat.com>
Acked-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>