udp: consistently apply ufo or fragmentation

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: e89e9cf5 ("[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>
Git-repo: https://git.kernel.org/pub/scm/linux/kernel/git/davem/net.git


Git-commit: 85f1bd9a7b5a79d5baa8bf44af19658f7bf77bfa
[srkupp@codeaurora.org: Resolved conflicts]
Change-Id: I6a2056d819bef06cff480b3104fa415fe7f9d079
Signed-off-by: Srinivasa Rao Kuppala <srkupp@codeaurora.org>