btrfs: fix compile when block cgroups are not enabled

bio->bi_css and bio->bi_ioc don't exist when block cgroups are not on.
This adds an ifdef around them. It's not perfect, but our
use of bi_ioc is being removed in the 4.3 merge window.

The bi_css usage really should go into bio_clone, but I want to make
sure that doesn't introduce problems for other bio_clone use cases.

Signed-off-by: Chris Mason <[e-mail]>

fs/btrfs/extent_io.c

···
 		btrfs_bio->csum = NULL;
 		btrfs_bio->csum_allocated = NULL;
 		btrfs_bio->end_io = NULL;
+
+#ifdef CONFIG_BLK_CGROUP
 		/* FIXME, put this into bio_clone_bioset */
 		if (bio->bi_css)
 			bio_associate_blkcg(new, bio->bi_css);
+#endif
 	}
 	return new;
 }

fs/btrfs/volumes.c

···
 	if (!bio)
 		return -ENOMEM;
 
+#ifdef CONFIG_BLK_CGROUP
 	if (first_bio->bi_ioc) {
 		get_io_context_active(first_bio->bi_ioc);
 		bio->bi_ioc = first_bio->bi_ioc;
···
 		css_get(first_bio->bi_css);
 		bio->bi_css = first_bio->bi_css;
 	}
+#endif
 	while (bvec <= (first_bio->bi_io_vec + first_bio->bi_vcnt - 1)) {
 		if (bio_add_page(bio, bvec->bv_page, bvec->bv_len,
 				 bvec->bv_offset) < bvec->bv_len) {