ipvs: Use outer header in ip_vs_bypass_xmit_v6

The ip_vs_iphdr may refer to an internal header, so use the outer one
instead.

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