nilfs2: convert checkpoint file to be folio-based

nilfs2: convert checkpoint file to be folio-based

Regarding the cpfile, a metadata file that manages checkpoints, convert
the page-based implementation to a folio-based implementation.

This change involves some helper functions to calculate byte offsets on
folios and removing a few helper functions that are no longer needed.

Link: https://lkml.kernel.org/r/20241024092602.13395-9-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>