udp: add gso support to virtual devices

Virtual devices such as tunnels and bonding can handle large packets.
Only segment packets when reaching a physical or loopback device.

Change-Id: I70a98ff455780a45ce3baea7b97e823716509802
Signed-off-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Git-commit: 83aa025f
Git-repo: https://git.kernel.org/pub/scm/linux/kernel/git/davem/net-next.git


Signed-off-by: Sean Tranchetti <stranche@codeaurora.org>