Currently, various functions are marked with the comment

  // May run without a P, so write barriers are not allowed.

However, "running without a P" is ambiguous. We intended these to mean
that m.p may be nil (which is the condition checked by the write
barrier). The comment could also be taken to mean that a
stop-the-world may happen, which is not the case for these functions
because they run in situations where there is in fact a function on
the stack holding a P locally, it just isn't in m.p.

Change these comments to state precisely what we mean, that m.p may be
nil.

Change-Id: I4a4a1d26aebd455e5067540e13b9f96a7482146c
Reviewed-on: https://go-review.googlesource.com/8209Reviewed-by: Minux Ma <minux@golang.org>
Reviewed-by: Rick Hudson <rlh@golang.org>