erofsutils: MountsToLayer slight optimizations

follow-up to 09f34d18b7

- Use strings.Cut instead of trimming prefixes and strings.Split
  to reduce allocations
- Use a switch for mount-type for slightly better readability
  than if / else if / else.
- Fix GoDoc to start with the function name.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

plugins/snapshots/erofs/erofsutils/mount_linux.go

@@ -55,7 +55,8 @@ func ConvertErofs(ctx context.Context, layerPath string, srcDir string, mkfsExtr
 	return nil
 }
 
-// Get the snapshot layer directory in order to generate EROFS-formatted blobs;
+// MountsToLayer returns the snapshot layer directory in order to generate
+// EROFS-formatted blobs;
 //
 // If mount[0].Type is `bind` or `erofs`, it just tries the source dir; Or if
 // mount[0].Type is `overlayfs`, it tries the parent of the upperdir;
@@ -64,25 +65,29 @@ func ConvertErofs(ctx context.Context, layerPath string, srcDir string, mkfsExtr
 // snapshot is really generated by the EROFS snapshotter instead of others.
 func MountsToLayer(mounts []mount.Mount) (string, error) {
 	var layer string
-	mnt := mounts[0]
-	if mnt.Type == "bind" || mnt.Type == "erofs" {
+	switch mnt := mounts[0]; mnt.Type {
+	case "bind", "erofs":
 		layer = filepath.Dir(mnt.Source)
-	} else if mnt.Type == "overlay" {
-		toplower := ""
+	case "overlay":
+		var topLower string
 		for _, o := range mnt.Options {
-			if strings.HasPrefix(o, "upperdir=") {
-				layer = filepath.Dir(strings.TrimPrefix(o, "upperdir="))
-			} else if strings.HasPrefix(o, "lowerdir=") {
-				toplower = filepath.Dir(strings.Split(strings.TrimPrefix(o, "lowerdir="), ":")[0])
+			if k, v, ok := strings.Cut(o, "="); ok {
+				switch k {
+				case "upperdir":
+					layer = filepath.Dir(v)
+				case "lowerdir":
+					dir, _, _ := strings.Cut(v, ":")
+					topLower = filepath.Dir(dir)
+				}
 			}
 		}
 		if layer == "" {
-			if toplower == "" {
+			if topLower == "" {
 				return "", fmt.Errorf("unsupported overlay layer for erofs differ: %w", errdefs.ErrNotImplemented)
 			}
-			layer = toplower
+			layer = topLower
 		}
-	} else {
+	default:
 		return "", fmt.Errorf("invalid filesystem type for erofs differ: %w", errdefs.ErrNotImplemented)
 	}
 	// If the layer is not prepared by the EROFS snapshotter, fall back to the next differ