*: fix data race on `enableTiFlashPoll` (#31820)
*: fix data race on `enableTiFlashPoll` (#31820)

close pingcap/tidb#31818
ngaut/tidb,c4pt0r/tidb,c4pt0r/tidb,pingcap/tidb,pingcap/tidb,kyle-wang/tidb,pingcap/tidb,pingcap/tidb,c4pt0r/tidb,kyle-wang/tidb,ngaut/tidb,kyle-wang/tidb,pingcap/tidb,ngaut/tidb,kyle-wang/tidb,kyle-wang/tidb,ngaut/tidb,c4pt0r/tidb,ngaut/tidb,kyle-wang/tidb,ngaut/tidb,c4pt0r/tidb,c4pt0r/tidb,pingcap/tidb