rcu: Disable interrupts directly in rcu_gp_init()

Interrupts are enabled in rcu_gp_init(), so this commit switches from
local_irq_save() and local_irq_restore() to local_irq_disable() and
local_irq_enable().

Link: https://lore.kernel.org/all/febb13ab-a4bb-48b4-8e97-7e9f7749e6da@moroto.mountain/



Reported-by: Dan Carpenter <dan.carpenter@linaro.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>