[NETFILTER]: Rename skb_ip_make_writable() to skb_make_writable()

There is nothing IPv4-specific in it.  In fact, it was already used by
IPv6, too...  Upcoming nfnetlink_queue code will use it for any kind
of packet.

Signed-off-by: Harald Welte <laforge@netfilter.org>
Signed-off-by: David S. Miller <davem@davemloft.net>