mm/damon: remove redundant page validation

It will never get a NULL page by pte_page() as discussed in thread [1],
thus remove the redundant page validation to fix below Smatch static
checker warning.

    mm/damon/vaddr.c:405 damon_hugetlb_mkold()
    warn: 'page' can't be NULL.

[1] https://lore.kernel.org/linux-mm/20220106091200.GA14564@kili/

Link: https://lkml.kernel.org/r/6d32f7d201b8970d53f51b6c5717d472aed2987c.1642386715.git.baolin.wang@linux.alibaba.com


Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: SeongJae Park <sj@kernel.org>
Acked-by: David Rientjes <rientjes@google.com>
Acked-by: Souptick Joarder <jrdr.linux@gmail.com>
Reviewed-by: Miaohe Lin <linmiaohe@huawei.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>