tsan: fix bug in shadow reset introduced in D128909

Correct a bug in the code that resets shadow memory introduced as part
of a previous change for the Go race detector (D128909). The bug was
that only the most recently added shadow segment was being reset, as
opposed to the entire extent of the segment created so far. This
fixes a bug identified in Google internal testing (b/240733951).

Differential Revision: https://reviews.llvm.org/D131256