tcp: create a helper to model exponential backoff

commit 01a523b0 upstream.

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>
Signed-off-by: Qiumiao Zhang <zhangqiumiao1@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>