blk-wbt: pass in enum wbt_flags to get_rq_wait()

This is in preparation for having more write queues, in which
case we would have needed to pass in more information than just
a simple 'is_kswapd' boolean.

Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Omar Sandoval <osandov@fb.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
diff --git a/block/blk-wbt.h b/block/blk-wbt.h
index a232c98..8038b4a 100644
--- a/block/blk-wbt.h
+++ b/block/blk-wbt.h
@@ -19,7 +19,9 @@ enum wbt_flags {
 };
 
 enum {
-	WBT_NUM_RWQ		= 2,
+	WBT_RWQ_BG		= 0,
+	WBT_RWQ_KSWAPD,
+	WBT_NUM_RWQ,
 };
 
 /*