udp: consistently apply ufo or fragmentation

udp: consistently apply ufo or fragmentation

Orabug: 26921303
CVE: CVE-2017-1000112

When iteratively building a UDP datagram with MSG_MORE and that
datagram exceeds MTU, consistently choose UFO or fragmentation.

Once skb_is_gso, always apply ufo. Conversely, once a datagram is
split across multiple skbs, do not consider ufo.

Sendpage already maintains the first invariant, only add the second.
IPv6 does not have a sendpage implementation to modify.

A gso skb must have a partial checksum, do not follow sk_no_check_tx
in udp_send_skb.

Found by syzkaller.

Fixes: e89e9cf539a2 ("[IPv4/IPv6]: UFO Scatter-gather approach")
Reported-by: Andrey Konovalov <andreyknvl@google.com>
Signed-off-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
(cherry picked from commit 85f1bd9a7b5a79d5baa8bf44af19658f7bf77bfa)
Signed-off-by: Brian Maly <brian.maly@oracle.com>
Conflicts:
	net/ipv4/ip_output.c
	net/ipv6/ip6_output.c
Signed-off-by: Dhaval Giani <dhaval.giani@oracle.com>