ipvs: Add hdr_flags to iphdr

These flags contain information like whether or not the addresses are
inverted or from icmp.  The first will allow us to drop an inverse param
all over the place, and the second will later be useful in scheduling icmp.

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