Currently, we only flush the per-P gcWork caches in gcMark, at the
beginning of mark termination. This is necessary to ensure that no
work is held up in these caches.

However, this flush happens after we update the GC controller state,
which depends on statistics about marked heap size and scan work that
are only updated by this flush. Hence, the controller is missing the
bulk of heap marking and scan work. This bug was introduced in commit
1b4025f4, which introduced the per-P gcWork caches.

Fix this by flushing these caches before we update the GC controller
state. We continue to flush them at the beginning of mark termination
as well to be robust in case any write barriers happened between the
previous flush and entering mark termination, but this should be a
no-op.

Change-Id: I8f0f91024df967ebf0c616d1c4f0c339c304ebaa
Reviewed-on: https://go-review.googlesource.com/9646Reviewed-by: Russ Cox <rsc@golang.org>