squashfs: use a folio throughout squashfs_read_folio()

Use modern folio APIs where they exist and convert back to struct
page for the internal functions.

Link: https://lkml.kernel.org/r/20241220224634.723899-1-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>