squashfs: convert squashfs_readpage_block() to take a folio

Remove a few accesses to page->mapping.

Link: https://lkml.kernel.org/r/20241220224634.723899-3-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>