rcu: Add missing annotation for exit_tasks_rcu_finish()

Sparse reports a warning at exit_tasks_rcu_finish(void)

|warning: context imbalance in exit_tasks_rcu_finish()
|- wrong count at exit

To fix this, this commit adds a __releases(&tasks_rcu_exit_srcu).
Given that exit_tasks_rcu_finish() does actually call __srcu_read_lock(),
this not only fixes the warning but also improves on the readability of
the code.
Signed-off-by: Jules Irenge <jbi.octave@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Reviewed-by: Joel Fernandes (Google) <joel@joelfernandes.org>

kernel/rcu/update.c

@@ -809,7 +809,7 @@ void exit_tasks_rcu_start(void) __acquires(&tasks_rcu_exit_srcu)
 }
 
 /* Do the srcu_read_unlock() for the above synchronize_srcu().  */
-void exit_tasks_rcu_finish(void)
+void exit_tasks_rcu_finish(void) __releases(&tasks_rcu_exit_srcu)
 {
 	preempt_disable();
 	__srcu_read_unlock(&tasks_rcu_exit_srcu, current->rcu_tasks_idx);