[release-branch.go1.15] runtime, time: disable preemption in addtimer

The timerpMask optimization updates a mask of Ps (potentially)
containing timers in pidleget / pidleput. For correctness, it depends on
the assumption that new timers can only be added to a P's own heap.

addtimer violates this assumption if it is preempted after computing pp.
That G may then run on a different P, but adding a timer to the original
P's heap.

Avoid this by disabling preemption while pp is in use.

Other uses of doaddtimer should be OK:

* moveTimers: always moves to the current P's heap
* modtimer, cleantimers, addAdjustedTimers, runtimer: does not add net
  new timers to the heap while locked

For #44868
Fixes #45731

Change-Id: I4a5d080865e854931d0a3a09a51ca36879101d72
Reviewed-on: https://go-review.googlesource.com/c/go/+/300610


Trust: Michael Pratt <mpratt@google.com>
Run-TryBot: Michael Pratt <mpratt@google.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-on: https://go-review.googlesource.com/c/go/+/313129


Trust: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Michael Pratt <mpratt@google.com>