printk: Don't allow cpu to get console lock during hotplugging

The flush of the console takes unnecessary time during the cpu
hotplug up operation.  It can delay the rcu synchronize_sched
operation, which in turn delays the hotplug operation. The flush
delays rcu synchronize_sched during scheduling domain creation
by up to 100 ms by interrupting the move to a quiescent state.
This change delays the flush of the console to later in the
notification chain of cpu_online. At the point in the cpu_up
operation where the flush now occurs, other tasks can already
be scheduled on the cpu that just came up.

Signed-off-by: Maya Spivak <mspivak@codeaurora.org>