IPVS: Add IPv6 support flag to schedulers

Add 'supports_ipv6' flag to struct ip_vs_scheduler to indicate whether a
scheduler supports IPv6. Set the flag to 1 in schedulers that work with
IPv6, 0 otherwise. This flag is checked in a later patch while trying to
add a service with a specific scheduler. Adjust debug in v6-supporting
schedulers to work with both address families.

Signed-off-by: Julius Volz <juliusv@google.com>
Signed-off-by: Simon Horman <horms@verge.net.au>