rcu: Add missing annotation for exit_tasks_rcu_start()

Sparse reports a warning at exit_tasks_rcu_start(void)

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

To fix this, this commit adds an __acquires(&tasks_rcu_exit_srcu).
Given that exit_tasks_rcu_start() 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>