fs/writeback: Convert inode_switch_wbs_work_fn to folios

This gets the statistics correct for large folios by modifying the
counters by the number of pages in the folio instead of by 1.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: William Kucharski <william.kucharski@oracle.com>