um: Remove the redundant newpage check in update_pte_range

The two checks have been identical since commit ef714f15 ("um:
remove force_flush_all from fork_handler"). And the inner one isn't
necessary anymore.

Signed-off-by: Tiwei Bie <tiwei.btw@antgroup.com>
Signed-off-by: Richard Weinberger <richard@nod.at>