squashfs: use folios in squashfs_bio_read_cached()

Remove an access to page->mapping and a few calls to the old page-based
APIs.  This doesn't support large folios, but it's still a nice
improvement.

Link: https://lkml.kernel.org/r/20250612143903.2849289-3-willy@infradead.org


Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Phillip Lougher <phillip@squashfs.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>