ipvs: uninitialized data with IP_VS_IPV6

The app_tcp_pkt_out() function expects "*diff" to be set and ends up
using uninitialized data if CONFIG_IP_VS_IPV6 is turned on.

The same issue is there in app_tcp_pkt_in().  Thanks to Julian Anastasov
for noticing that.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Julian Anastasov <ja@ssi.bg>
Signed-off-by: Simon Horman <horms@verge.net.au>