Merge branch 'tcp-take-care-of-empty-skbs-in-write-queue'

Eric Dumazet says:
====================
tcp: take care of empty skbs in write queue

We understood recently that TCP sockets could have an empty
skb at the tail of the write queue, leading to various problems.

This patch series :

1) Make sure we do not send an empty packet since this
   was unintended and causing crashes in old kernels.

2) Change tcp_write_queue_empty() to not be fooled by
   the presence of an empty skb.

3) Fix a bug that could trigger suboptimal epoll()
   application behavior under memory pressure.
====================

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>