ipvs: convert the IP_VS_XMIT macros to functions

It was a bad idea to hide return statements in macros.

Signed-off-by: Julian Anastasov <ja@ssi.bg>
Signed-off by: Hans Schillstrom <hans@schillstrom.com>
Signed-off-by: Simon Horman <horms@verge.net.au>