srcu,notifier: Remove #ifdefs in favor of SRCU Tiny srcu_usage

This commit removes two #ifdef directives from include/linux/notifier.h
by causing SRCU Tiny to provide a dummy srcu_usage structure and a dummy
__SRCU_USAGE_INIT() macro.

Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Matthias Brugger <matthias.bgg@gmail.com>
Cc: "Rafael J. Wysocki" <rafael.j.wysocki@intel.com>
Cc: "Michał Mirosław" <mirq-linux@rere.qmqm.pl>
Cc: Dmitry Osipenko <dmitry.osipenko@collabora.com>
Cc: Sachin Sant <sachinp@linux.ibm.com>
Cc: Joel Fernandes (Google) <joel@joelfernandes.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>