ipvs: support ipv4 in ipv6 and ipv6 in ipv4 tunnel forwarding

Pull the common logic for preparing an skb to prepend the header into a
single function and then set fields such that they can be used in either
case (generalize tos and tclass to dscp, hop_limit and ttl to ttl, etc)

Signed-off-by: Alex Gartrell <agartrell@fb.com>
Acked-by: Julian Anastasov <ja@ssi.bg>
Signed-off-by: Simon Horman <horms@verge.net.au>