nilfs2: convert common metadata file code to be folio-based

nilfs2: convert common metadata file code to be folio-based

In the common routines for metadata files, nilfs_mdt_insert_new_block(),
which inserts a new block buffer into the cache, is still page-based, and
there are two places where bh_offset() is used.  Convert these to
page-based.

Link: https://lkml.kernel.org/r/20241024092602.13395-3-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>