Write barriers in gcFlushBgCredit lead to very subtle bugs because it
executes after the getfull barrier. I tracked some bugs of this form
down before go:nowritebarrierrec was implemented. Ensure that they
don't reappear by making gcFlushBgCredit go:nowritebarrierrec.

Change-Id: Ia5ca2dc59e6268bce8d8b4c87055bd0f6e19bed2
Reviewed-on: https://go-review.googlesource.com/17052Reviewed-by: Russ Cox <rsc@golang.org>