writeback: add wbc_to_write_flags()

Add wbc_to_write_flags(), which returns the write modifier flags to use,
based on a struct writeback_control. No functional changes in this
patch, but it prepares us for factoring other wbc fields for write type.

Signed-off-by: Jens Axboe <axboe@fb.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Reviewed-by: Christoph Hellwig <hch@lst.de>
diff --git a/mm/page_io.c b/mm/page_io.c
index a2651f5..23f6d0d 100644
--- a/mm/page_io.c
+++ b/mm/page_io.c
@@ -320,10 +320,7 @@ int __swap_writepage(struct page *page, struct writeback_control *wbc,
 		ret = -ENOMEM;
 		goto out;
 	}
-	if (wbc->sync_mode == WB_SYNC_ALL)
-		bio_set_op_attrs(bio, REQ_OP_WRITE, REQ_SYNC);
-	else
-		bio_set_op_attrs(bio, REQ_OP_WRITE, 0);
+	bio->bi_opf = REQ_OP_WRITE | wbc_to_write_flags(wbc);
 	count_vm_event(PSWPOUT);
 	set_page_writeback(page);
 	unlock_page(page);