tcp: create a helper to model exponential backoff

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

Change-Id: I2a95973a9d906b91e4c6adb483dd280d2c66ed65
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>
Git-Commit: 01a523b071618abbc634d1958229fe3bd2dfa5fa
Git-repo: https://android.googlesource.com/kernel/common/


Signed-off-by: Kaustubh Pandey <kapandey@codeaurora.org>