tcp: properly reset sackhint values when SACK recovery is done

When the SACK scoreboard collapses, properly clear all the counters.
The counters are used in tcp_compute_pipe(), which can be called
anytime later after the SACK recovery. The returned result can be
totally bogus, including both too large and too small values.

PR: 283649
Reviewed by: rscheff
Differential Revision: https://reviews.freebsd.org/D48236