nilfs2: convert DAT file to be folio-based

Regarding the DAT, a metadata file that manages virtual block addresses,
convert the page-based implementation to a folio-based implementation.

Link: https://lkml.kernel.org/r/20241024092602.13395-7-konishi.ryusuke@gmail.com


Signed-off-by: Ryusuke Konishi <konishi.ryusuke@gmail.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>