tcp: create a helper to model exponential backoff

mainline inclusion
from mainline-v5.1-rc1
commit 01a523b0
category: bugfix
bugzilla:
https://gitee.com/openeuler/kernel/issues/I4AFRJ?from=project-issue


CVE: NA

------------------------------------------------------------

Create a helper to model TCP exponential backoff for the next patch.
This is pure refactor w no behavior change.

Signed-off-by: Yuchung Cheng <ycheng@google.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Neal Cardwell <ncardwell@google.com>
Reviewed-by: Soheil Hassas Yeganeh <soheil@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Conflicts:
        net/ipv4/tcp_timer.c
Signed-off-by: Jiazhenyuan <jiazhenyuan@uniontech.com> #openEuler_contributor
Signed-off-by: Laibin Qiu <qiulaibin@huawei.com>
Reviewed-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>