bcache/md: Use raid stripe size

Now that we've got code for raid5/6 stripe awareness, bcache just needs
to know about the stripes and when writing partial stripes is expensive
- we probably don't want to enable this optimization for raid1 or 10,
even though they have stripes. So add a flag to queue_limits.

Signed-off-by: Kent Overstreet <kmo@daterainc.com>

include/linux/blkdev.h

@@ -291,6 +291,7 @@ struct queue_limits {
 	unsigned char		discard_misaligned;
 	unsigned char		cluster;
 	unsigned char		discard_zeroes_data;
+	unsigned char		raid_partial_stripes_expensive;
 };
 
 struct request_queue {