selftests: net: test IPV6_HOPLIMIT

Test setting IPV6_HOPLIMIT via setsockopt and cmsg
across socket types.

Output without the kernel support (this series):

  Case HOPLIMIT ICMP cmsg - packet data returned 1, expected 0
  Case HOPLIMIT ICMP diff - packet data returned 1, expected 0

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>