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/relocation.c b/fs/btrfs/relocation.c
index 37175ace..e2cf65c 100644
--- a/fs/btrfs/relocation.c
+++ b/fs/btrfs/relocation.c
@@ -4334,7 +4334,7 @@
 	rc->block_group = btrfs_lookup_block_group(fs_info, group_start);
 	BUG_ON(!rc->block_group);
 
-	ret = btrfs_inc_block_group_ro(extent_root, rc->block_group);
+	ret = btrfs_inc_block_group_ro(fs_info, rc->block_group);
 	if (ret) {
 		err = ret;
 		goto out;