ipset: support creation of inet6 sets

[stv0g]

This PR adds support for AF_INET6 address and network management in the ipset code.

[vishvananda]

there are multiple mistakes in fallthrough in this pr. Please fix and ammedn the commit. Thanks!

[stv0g]

Hi @vishvananda,

sorry, the fallthroughs are fixed now.

[vishvananda]

Looks like the tests are failing in CI. Does it need a newer kernel version? or a certain kernel module?

[noblehng]

I need this and tested it. It is very strange that just adding any constant in nl/ipset_linux.go will fail the exact same test without any other change. The test is just testing parsing of a fixture.

Edit:
What is more funny is that if I move the added constants to its own group, the test passed. Something like:

const (
        IPSET_ATTR_IPV4 = 1 
        IPSET_ATTR_IPV6 = 2 
)

One more thing, the constants should be named by IPSET_ATTR_IPADDR_IPV4 and IPSET_ATTR_IPADDR_IPV6 to match linux header.

Edit2:
Oh, it was because the following iota got shift by two:

IPSET_ATTR_GC = 3 + iota

[aboch]

@stv0g