squashfs; convert squashfs_copy_cache() to take a folio

Remove accesses to page->index and page->mapping.  Also use folio
APIs where available.  This code still assumes order 0 folios.

[dan.carpenter@linaro.org: fix a NULL vs IS_ERR() bug]
  Link: https://lkml.kernel.org/r/7b7f44d6-9153-4d7c-b65b-2d78febe6c7a@stanley.mountain
Link: https://lkml.kernel.org/r/20241220224634.723899-4-willy@infradead.org


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