netfilter: ipvs: prefer skb_ensure_writable

It does the same thing, use it instead so we can remove skb_make_writable.

Signed-off-by: Florian Westphal <fw@strlen.de>
Acked-by: Simon Horman <horms@verge.net.au>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>