mm/khugepaged: convert hpage_collapse_scan_pmd() to use folios

Replaces 5 calls to compound_head(), and removes 1385 bytes of kernel
text.

Link: https://lkml.kernel.org/r/20231020183331.10770-3-vishal.moola@gmail.com


Signed-off-by: Vishal Moola (Oracle) <vishal.moola@gmail.com>
Reviewed-by: Rik van Riel <riel@surriel.com>
Reviewed-by: Yang Shi <shy828301@gmail.com>
Cc: Kefeng Wang <wangkefeng.wang@huawei.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>