lnutils: add createdir util function

This utility function replaces repetitive logic patterns
throughout LND.

lnutils/fs.go

@@ -0,0 +1,31 @@
+package lnutils
+
+import (
+	"errors"
+	"fmt"
+	"os"
+)
+
+// CreateDir creates a directory if it doesn't exist and also handles
+// symlink-related errors with user-friendly messages. It creates all necessary
+// parent directories with the specified permissions.
+func CreateDir(dir string, perm os.FileMode) error {
+	err := os.MkdirAll(dir, perm)
+	if err == nil {
+		return nil
+	}
+
+	// Show a nicer error message if it's because a symlink
+	// is linked to a directory that does not exist
+	// (probably because it's not mounted).
+	var pathErr *os.PathError
+	if errors.As(err, &pathErr) && os.IsExist(err) {
+		link, lerr := os.Readlink(pathErr.Path)
+		if lerr == nil {
+			return fmt.Errorf("is symlink %s -> %s "+
+				"mounted?", pathErr.Path, link)
+		}
+	}
+
+	return fmt.Errorf("failed to create directory '%s': %w", dir, err)
+}