mm/page_alloc: update comments in __free_pages_ok()

Add a comment to explain why we call get_pfnblock_migratetype() twice in
__free_pages_ok().

Link: https://lkml.kernel.org/r/20221201135045.31663-1-wonder_rock@126.com


Signed-off-by: Deyan Wang <wonder_rock@126.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>