printk: Don't take console semaphore in atomic context

The CPU HOTPLUG take_cpu_down path is invokved with preemption
disabled via stop_machine. This causes a "Scheduling while
atomic" BUG when there is contention for the console semaphore.
The solution is to defer the console flush until it's not in
scheduling violation.

Change-Id: I2d0d58576a4db308ee40850a18a6bb9784ca4e4b
Signed-off-by: Michael Bohan <mbohan@codeaurora.org>
(cherry picked from commit f6d11b2eb9c110d0801aa40b1bfdb8194a5e3e3a)