nilfs2: convert metadata aops from writepage to writepages

By implementing ->writepages instead of ->writepage, we remove a layer of
indirect function calls from the writeback path and the last use of struct
page in nilfs2.

[konishi.ryusuke@gmail.com: fixed panic by using buffer_migrate_folio_norefs]
Link: https://lkml.kernel.org/r/20241002150036.1339475-5-willy@infradead.org
Link: https://lkml.kernel.org/r/20241024092602.13395-13-konishi.ryusuke@gmail.com


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