btrfs: Remove a use of PAGE_SIZE in btrfs_invalidate_folio()

While btrfs doesn't use large folios yet, this should have been changed
as part of the conversion from invalidatepage to invalidate_folio.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Al Viro <viro@zeniv.linux.org.uk>
Acked-by: Al Viro <viro@zeniv.linux.org.uk>