sftp: terminate the missing-ancestor permission walk at the root

A path whose ancestors are all absent recursed on the root forever,
since Dir("/") is "/".

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Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

weed/sftpd/sftp_permissions.go

@@ -67,6 +67,9 @@ func (fs *SftpServer) CheckFilePermission(path string, perm string) error {
 		// check permissions on the parent directory instead
 		if err == os.ErrNotExist {
 			parentPath := stdpath.Dir(path)
+			if parentPath == path {
+				return os.ErrPermission
+			}
 			// Check if user can write to the parent directory
 			return fs.CheckFilePermission(parentPath, perm)
 		}