tcp rack: fix sending

In rack_output(), idle is used as a boolean variable. So don't use it
as an int and don't clear it afterwards.
This avoids setting idle to false, when it is not intended.

Reported by: olivier
Reviewed by: rrs, rscheff
Sponsored by: Netflix, Inc.
Differential Revision: https://reviews.freebsd.org/D44610

(cherry picked from commit 7df0ef5f48e1c67b3f1df7c7964bfa59bc56f4e4)