mm/damon/paddr: initialize 'folio' variables to NULL for clarity

mm/damon/paddr: initialize 'folio' variables to NULL for clarity

In damon_pa_mark_accessed_or_deactivate(), damon_pa_pageout(),
damon_pa_migrate(), and damon_pa_stat(), the local variable 'folio' is
declared but not initialized.

Initialize 'folio' to NULL to improve code readability and
maintainability.

Link: https://patch.msgid.link/20260104013255.16962-1-yangqixiao@inspur.com
Link: https://lkml.kernel.org/r/20260108013041.80601-1-sj@kernel.org
Signed-off-by: Aaron Yang <yangqixiao@inspur.com>
Reviewed-by: SeongJae Park <sj@kernel.org>
Signed-off-by: SeongJae Park <sj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>