Test TIMESTAMPING and TXTIME across UDP / ICMP and IP versions.

Before ICMPv6 support:
  # ./tools/testing/selftests/net/cmsg_time.sh
    Case ICMPv6  - ts cnt returned '0', expected '2'
    Case ICMPv6  - ts0 SCHED returned '', expected 'OK'
    Case ICMPv6  - ts0 SND returned '', expected 'OK'
    Case ICMPv6  - TXTIME abs returned '', expected 'OK'
    Case ICMPv6  - TXTIME rel returned '', expected 'OK'
  FAIL - 5/36 cases failed

After:
  # ./tools/testing/selftests/net/cmsg_time.sh
  OK
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>