IPVS: use z modifier for sizeof() argument

Reported-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Simon Horman <horms@verge.net.au>
Acked-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Hans Schillstrom <hans@schillstrom.com>
Tested-by: Hans Schillstrom <hans@schillstrom.com>
Signed-off-by: Patrick McHardy <kaber@trash.net>