fit: optimize the selection of witness rule candidates (#5904)

ref #5568

Signed-off-by: Wenbo Zhang <ethercflow@gmail.com>

Co-authored-by: Ti Chi Robot <ti-community-prow-bot@tidb.io>

server/schedule/placement/fit.go

@@ -243,9 +243,10 @@ func (w *fitWorker) fitRule(index int) bool {
 		// Only consider stores:
 		// 1. Match label constraints
 		// 2. Role match, or can match after transformed.
-		// 3. Not selected by other rules.
+		// 3. Don't select leader as witness.
+		// 4. Not selected by other rules.
 		for _, p := range w.peers {
-			if !p.selected && MatchLabelConstraints(p.store, w.rules[index].LabelConstraints) {
+			if !p.selected && MatchLabelConstraints(p.store, w.rules[index].LabelConstraints) && !(p.isLeader && w.supportWitness && w.rules[index].IsWitness) {
 				candidates = append(candidates, p)
 			}
 		}