selftests: net: test IPV6_DONTFRAG

Test setting IPV6_DONTFRAG via setsockopt and cmsg
across socket types.

Output without the kernel support (this series):

    Case DONTFRAG ICMP setsock returned 0, expected 1
    Case DONTFRAG ICMP cmsg returned 0, expected 1
    Case DONTFRAG ICMP both returned 0, expected 1
    Case DONTFRAG ICMP diff returned 0, expected 1
  FAIL - 4/24 cases failed

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