[IPv4] UFO: prevent generation of chained skb destined to UFO device

Problem: ip_append_data() could wrongly generate a chained skb for
devices which support UFO.  When sk_write_queue is not empty
(e.g. MSG_MORE), __instead__ of appending data into the next nr_frag
of the queued skb, a new chained skb is created.

I would normally assume UFO device should get data in nr_frags and not
in frag_list.  Later the udp4_hwcsum_outgoing() resets csum to NONE
and skb_gso_segment() has oops.

Proposal:
1. Even length is less than mtu, employ ip_ufo_append_data()
and append data to the __existed__ skb in the sk_write_queue.

2. ip_ufo_append_data() is fixed due to a wrong manipulation of
peek-ing and later enqueue-ing of the same skb.  Now, enqueuing is
always performed, because on error the further
ip_flush_pending_frames() would release the queued skb.

Signed-off-by: Kostya B <bkostya@hotmail.com>
Acked-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>