writeback: move the folio_prepare_writeback loop out of write_cache_pages()

Move the loop for should-we-write-this-folio to writeback_get_folio.

[hch@lst.de: fold loop into existing helper instead of a separate one per Jan]
Link: https://lkml.kernel.org/r/20240215063649.2164017-13-hch@lst.de


Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Acked-by: Dave Chinner <dchinner@redhat.com>
Cc: Christian Brauner <brauner@kernel.org>
Cc: David Howells <dhowells@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>