btrfs: rename workspaces_list to workspace_manager

This is in preparation for zstd compression levels. As each level will
require different size of workspace, workspaces_list is no longer a
really fitting name.

Reviewed-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Dennis Zhou <dennis@kernel.org>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>