tcp: Rack at times can miscalculate the RTT from what it thinks is a persists probe respone.

Turns out that if a peer sends in a window update right after rack fires off
a persists probe, we can mis-interpret the window update and calculate
a bogus RTT (very short). We still process the window update and send
the data but we incorrectly generate an RTT. We should be only doing
the RTT stuff if the rwnd is still small and has not changed.

Reviewed by: Michael Tuexen
Sponsored by: Netflix Inc.
Differential Revision: https://reviews.freebsd.org/D32717