btrfs: convert btrfs_inc_block_group_ro to accept fs_info

btrfs_inc_block_group_ro is either passed the extent root or the dev
root, but it doesn't do anything with the dev tree.  Let's convert
to passing an fs_info and using the extent root.

Signed-off-by: Jeff Mahoney <jeffm@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>
diff --git a/fs/btrfs/scrub.c b/fs/btrfs/scrub.c
index f7dffac..ff9a11c 100644
--- a/fs/btrfs/scrub.c
+++ b/fs/btrfs/scrub.c
@@ -3582,7 +3582,7 @@ int scrub_enumerate_chunks(struct scrub_ctx *sctx,
 		 * -> btrfs_scrub_pause()
 		 */
 		scrub_pause_on(fs_info);
-		ret = btrfs_inc_block_group_ro(root, cache);
+		ret = btrfs_inc_block_group_ro(fs_info, cache);
 		if (!ret && is_dev_replace) {
 			/*
 			 * If we are doing a device replace wait for any tasks