kthread/smpboot: Disable irq while setting smpboot thread as running

While setting smpboot thread state as running, there is possibility
of irq may fire at same core and wake up the smpboot thread of same 
core which create self deadlock. To avoid that, protect the
same with spin_lock_irqsave.

Change-Id: I5eca9b27af94fee22af3bb201f26b63ed8930efe
Signed-off-by: Gaurav Kohli <gkohli@codeaurora.org>