devfs - sync devfs after cloning a device

devfs - sync devfs after cloning a device

 * Use case are devices such as tty_pty which create a ptm/foo node as
   the cloned one, but also create a slave node (in pts/foo).

 * Without this, it's possible that ptm/foo is created before pts/foo
   is, and the program may race and not see pts/foo yet.