fix pre-allocation computation in dentry resolver (#21946)

[CWS] fix pre-allocation computation in dentry resolver

pkg/security/resolvers/dentry/resolver.go

@@ -329,13 +329,15 @@ func computeFilenameFromParts(parts []string) string {
 		return "/"
 	}
 
-	var builder strings.Builder
-
 	// pre-allocation
+	prealloc := 0
 	for _, part := range parts {
-		builder.Grow(len(part) + 1)
+		prealloc += len(part) + 1
 	}
 
+	var builder strings.Builder
+	builder.Grow(prealloc)
+
 	// reverse iteration
 	for i := 0; i < len(parts); i++ {
 		j := len(parts) - 1 - i