Revert "drivers/net, ipv6: Select IPv6 fragment idents for virtio UFO packets"

Revert "drivers/net, ipv6: Select IPv6 fragment idents for virtio UFO packets"

This reverts commit 5188cd44c55db3e92cd9e77a40b5baa7ed4340f7.

Now that GSO layer can track if fragment id has been selected
and can allocate one if necessary, we don't need to do this in
tap and macvtap.  This reverts most of the code and only keeps
the new ipv6 fragment id generation function that is still needed.

Fixes: 3d0ad09412ff (drivers/net: Disable UFO through virtio)
Signed-off-by: Vladislav Yasevich <vyasevic@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>