fil_space_t::n_pending_ops, n_pending_ios: Use a combination of
fil_system.mutex and atomic memory access for protection.

fil_space_t::release(): Replaces fil_space_release().
Does not acquire fil_system.mutex.

fil_space_t::release_for_io(): Replaces fil_space_release_for_io().
Does not acquire fil_system.mutex.