mm/damon: validate if the pmd entry is present before accessing

mm/damon: validate if the pmd entry is present before accessing

The pmd_huge() is used to validate if the pmd entry is mapped by a huge
page, also including the case of non-present (migration or hwpoisoned) pmd
entry on arm64 or x86 architectures.  Thus we should validate if it is
present before making the pmd entry old or getting young state, otherwise
we can not get the correct corresponding page.

Link: https://lkml.kernel.org/r/2838b6737bc259cf575ff11fd1c4b7fdb340fa73.1660717122.git.baolin.wang@linux.alibaba.com
Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: SeongJae Park <sj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>