notifier: Add atomic_notifier_call_chain_is_empty()

Add atomic_notifier_call_chain_is_empty() that returns true if given
atomic call chain is empty.

The first user of this new notifier API function will be the kernel
power-off core code that will support power-off call chains. The core
code will need to check whether there is a power-off handler registered
at all in order to decide whether to halt machine or power it off.

Reviewed-by: Michał Mirosław <mirq-linux@rere.qmqm.pl>
Signed-off-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>