selftests/net: ipv6 flowlabel

Test the IPv6 flowlabel control and datapath interfaces:

Acquire and release the right to use flowlabels with socket option
IPV6_FLOWLABEL_MGR.

Then configure flowlabels on send and read them on recv with cmsg
IPV6_FLOWINFO. Also verify auto-flowlabel if not explicitly set.

This helped identify the issue fixed in commit 95c16925

 ("ipv6:
invert flowlabel sharing check in process and user mode")

Signed-off-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>