kcsan: Add kcsan_set_access_mask() support

When setting up an access mask with kcsan_set_access_mask(), KCSAN will
only report races if concurrent changes to bits set in access_mask are
observed. Conveying access_mask via a separate call avoids introducing
overhead in the common-case fast-path.

Acked-by: John Hubbard <jhubbard@nvidia.com>
Signed-off-by: Marco Elver <elver@google.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>

init/init_task.c

@@ -167,6 +167,7 @@ struct task_struct init_task
 		.atomic_next		= 0,
 		.atomic_nest_count	= 0,
 		.in_flat_atomic		= false,
+		.access_mask		= 0,
 	},
 #endif
 #ifdef CONFIG_TRACE_IRQFLAGS