rcu: Keep synchronize_rcu() from enabling irqs in early boot

Making polled RCU grace periods account for expedited grace periods
required acquiring the leaf rcu_node structure's lock during early boot,
but after rcu_init() was called.  This lock is irq-disabled, but the
code incorrectly assumes that irqs are always disabled when invoking
synchronize_rcu().  The exception is early boot before the scheduler has
started, which means that upon return from synchronize_rcu(), irqs will
be incorrectly enabled.

This commit fixes this bug by using irqsave/irqrestore locking primitives.

Fixes: bf95b2bc ("rcu: Switch polled grace-period APIs to ->gp_seq_polled")

Reported-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>