linux/getroot: fix descriptor leak.

Found by: Coverity scan.

grub-core/osdep/linux/getroot.c

@@ -360,8 +360,10 @@ get_btrfs_fs_prefix (const char *mount_path)
 	ret[1+namelen] = '\0';
     }
   if (!ret)
-    return xstrdup ("/");
+    ret = xstrdup ("/");
+  close (fd);
   return ret;
+
  fail:
   free (ret);
   close (fd);