mm: damon must atomically clear young on ptes and pmds

It is racy to non-atomically read a pte, then clear the young bit, then
write it back as this could discard dirty information. Further, it is
bad practice to directly set a pte entry within a table. Instead
clearing young must go through the arch-provided helper,
ptep_test_and_clear_young() to ensure it is modified atomically and to
give the arch code visibility and allow it to validate (and potentially
modify) the operation.

Fixes: 46c3a0ac ("mm/damon/vaddr: separate commonly usable functions")
Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>