Btrfs: use bio_clone_fast to clone our bio

For raid1 and raid10, we clone the original bio to the bios which are then
sent to different disks.

Right now we use bio_clone_bioset to create a clone bio with iterating
bi_io_vec to initialize it.  This changes it to use bio_clone_fast()
which creates a clone bio but only copies the bi_io_vec pointer
instead of iterating bi_io_vec.

Signed-off-by: Liu Bo <bo.li.liu@oracle.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/extent_io.c

@@ -2694,7 +2694,7 @@ struct bio *btrfs_bio_clone(struct bio *bio, gfp_t gfp_mask)
 	struct btrfs_io_bio *btrfs_bio;
 	struct bio *new;
 
-	new = bio_clone_bioset(bio, gfp_mask, btrfs_bioset);
+	new = bio_clone_fast(bio, gfp_mask, btrfs_bioset);
 	if (new) {
 		btrfs_bio = btrfs_io_bio(new);
 		btrfs_bio->csum = NULL;