rcu: Move callback-list warning to irq-disable region

rcu: Move callback-list warning to irq-disable region

After adopting callbacks from a newly offlined CPU, the adopting CPU
checks to make sure that its callback list's count is zero only if the
list has no callbacks and vice versa.  Unfortunately, it does so after
enabling interrupts, which means that false positives are possible due to
interrupt handlers invoking call_rcu().  Although these false positives
are improbable, rcutorture did make it happen once.

This commit therefore moves this check to an irq-disabled region of code,
thus suppressing the false positive.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>