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/[email protected]
Signed-off-by: Ryusuke Konishi <[email protected]>
Cc: Matthew Wilcox (Oracle) <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

Author: konis