tcp: do not send empty skb from tcp_write_xmit()

[ Upstream commit 1f85e6267caca44b30c54711652b0726fadbb131 ]

Backport of commit fdfc5c8594c2 ("tcp: remove empty skb from
write queue in error cases") in linux-4.14 stable triggered
various bugs. One of them has been fixed in commit ba2ddb43f270
("tcp: Don't dequeue SYN/FIN-segments from write-queue"), but
we still have crashes in some occasions.

Root-cause is that when tcp_sendmsg() has allocated a fresh
skb and could not append a fragment before being blocked
in sk_stream_wait_memory(), tcp_write_xmit() might be called
and decide to send this fresh and empty skb.

Sending an empty packet is not only silly, it might have caused
many issues we had in the past with tp->packets_out being
out of sync.

Fixes: c65f7f00 ("[TCP]: Simplify SKB data portion allocation with NETIF_F_SG.")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Christoph Paasch <cpaasch@apple.com>
Acked-by: Neal Cardwell <ncardwell@google.com>
Cc: Jason Baron <jbaron@akamai.com>
Acked-by: Soheil Hassas Yeganeh <soheil@google.com>
Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>