[release-branch.go1.16] runtime: in adjustTimers back up as far as necessary

When the adjustTimers function removed a timer it assumed it was
sufficient to continue the heap traversal at that position.
However, in some cases a timer will be moved to an earlier
position in the heap. If that timer is timerModifiedEarlier,
that can leave timerModifiedEarliest not correctly representing
the earlier such timer.

Fix the problem by restarting the heap traversal at the earliest
changed position.

For #47762
Fixes #47858

Change-Id: I152bbe62793ee40a680baf49967bcb89b1f94764
Reviewed-on: https://go-review.googlesource.com/c/go/+/343882


Trust: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
(cherry picked from commit 2da3375e)
Reviewed-on: https://go-review.googlesource.com/c/go/+/350000