squashfs: convert squashfs_fill_page() to take a folio

squashfs_fill_page is only used in this file, so make it static.
Use kmap_local instead of kmap_atomic, and return a bool so that
the caller can use folio_end_read() which saves an atomic operation
over calling folio_mark_uptodate() followed by folio_unlock().

[willy@infradead.org: fix polarity of "uptodate" Thanks to Ryan for testing]
  Link: https://lkml.kernel.org/r/20250110163300.3346321-2-willy@infradead.org
Link: https://lkml.kernel.org/r/20241220224634.723899-5-willy@infradead.org


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