Btrfs: cleanup: use consistent lock naming

It confuses Smatch that we use two names for the same lock.  Plus the
shorter name is nicer.  This doesn't change how the code works, it's
just a cleanup.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>

fs/btrfs/extent-tree.c

@@ -3578,7 +3578,7 @@ again:
 	space_info->chunk_alloc = 0;
 	spin_unlock(&space_info->lock);
 out:
-	mutex_unlock(&extent_root->fs_info->chunk_mutex);
+	mutex_unlock(&fs_info->chunk_mutex);
 	return ret;
 }