md: fast clone bio in bio_clone_mddev()

Firstly bio_clone_mddev() is used in raid normal I/O and isn't
in resync I/O path.

Secondly all the direct access to bvec table in raid happens on
resync I/O except for write behind of raid1, in which we still
use bio_clone() for allocating new bvec table.

So this patch replaces bio_clone() with bio_clone_fast()
in bio_clone_mddev().

Also kill bio_clone_mddev() and call bio_clone_fast() directly, as
suggested by Christoph Hellwig.

Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Ming Lei <tom.leiming@gmail.com>
Signed-off-by: Shaohua Li <shli@fb.com>
diff --git a/drivers/md/raid5.c b/drivers/md/raid5.c
index e2fa88f..b193316 100644
--- a/drivers/md/raid5.c
+++ b/drivers/md/raid5.c
@@ -5056,9 +5056,9 @@
 		return 0;
 	}
 	/*
-	 * use bio_clone_mddev to make a copy of the bio
+	 * use bio_clone_fast to make a copy of the bio
 	 */
-	align_bi = bio_clone_mddev(raid_bio, GFP_NOIO, mddev);
+	align_bi = bio_clone_fast(raid_bio, GFP_NOIO, mddev->bio_set);
 	if (!align_bi)
 		return 0;
 	/*