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/fs/gfs2/meta_io.c b/fs/gfs2/meta_io.c
index e562b11..49db8ef 100644
--- a/fs/gfs2/meta_io.c
+++ b/fs/gfs2/meta_io.c
@@ -37,8 +37,7 @@
 {
 	struct buffer_head *bh, *head;
 	int nr_underway = 0;
-	int write_flags = REQ_META | REQ_PRIO |
-		(wbc->sync_mode == WB_SYNC_ALL ? REQ_SYNC : 0);
+	int write_flags = REQ_META | REQ_PRIO | wbc_to_write_flags(wbc);
 
 	BUG_ON(!PageLocked(page));
 	BUG_ON(!page_has_buffers(page));