tcp: fix possible socket leaks in internal pacing mode

This patch is addressing an issue in stable linux-4.19 only.

In linux-4.20, TCP stack adopted EDT (Earliest Departure
Time) model and this issue was incidentally fixed.

Issue at hand was an extra sock_hold() from tcp_internal_pacing()
in paths not using tcp_xmit_retransmit_queue()

Jason Xing reported this leak and provided a patch stopping
the extra sock_hold() to happen.

This patch is more complete and makes sure to avoid
unnecessary extra delays, by reprogramming the high
resolution timer.

Fixes: 73a6bab5 ("tcp: switch pacing timer to softirq based hrtimer")
Reference: https://lore.kernel.org/all/CANn89i+7-wE4xr5D9DpH+N-xkL1SB8oVghCKgz+CT5eG1ODQhA@mail.gmail.com/


Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: Jason Xing <kerneljasonxing@gmail.com>
Reported-by: Zhang Changzhong <zhangchangzhong@huawei.com>
Cc: liweishi <liweishi@kuaishou.com>
Cc: Shujin Li <lishujin@kuaishou.com>
Cc: Neal Cardwell <ncardwell@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>