replica_rac2: remove unlocked pending updates read

In `ProcessPiggybackedAdmittedAtLeaderRaftMuLocked`, we swap the
pending updates map with another one before releasing the mutex, in
order to minimize the time holding a mutex. Subsequently, the method
would then check if the updates map was empty by calling `len()`,
performing a read.

When the updates map was not empty, the read was against a map which was
now only referenced in this function, because a new map was swapped in
under the lock.

When the updates map was empty, the map isn't swapped out and is
referenced by the receiver struct instance, reading a ref without
locking.

Return whether the updates map was empty, to avoid racing when reading
the length of the shared ref empty map.

Part of: #130187
Release note: None

pkg/kv/kvserver/kvflowcontrol/replica_rac2/processor.go

@@ -1022,15 +1022,16 @@ func (p *processorImpl) ProcessPiggybackedAdmittedAtLeaderRaftMuLocked(ctx conte
 	var updates map[roachpb.ReplicaID]rac2.AdmittedVector
 	// Swap the updates map with the empty scratch. This is an optimization to
 	// minimize the time we hold the pendingAdmittedMu lock.
-	func() {
+	if updatesEmpty := func() bool {
 		p.leader.pendingAdmittedMu.Lock()
 		defer p.leader.pendingAdmittedMu.Unlock()
 		if updates = p.leader.pendingAdmittedMu.updates; len(updates) != 0 {
 			p.leader.pendingAdmittedMu.updates = p.leader.scratch
 			p.leader.scratch = updates
+			return false
 		}
-	}()
-	if len(updates) == 0 {
+		return true
+	}(); updatesEmpty {
 		return
 	}
 	for replicaID, state := range updates {