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 <clm@fb.com>
diff --git a/fs/btrfs/extent_io.c b/fs/btrfs/extent_io.c
index 3cfbd62..fa19f2f 100644
--- a/fs/btrfs/extent_io.c
+++ b/fs/btrfs/extent_io.c
@@ -2730,9 +2730,12 @@
 		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;
 }