arch/x86/kernel/smpboot.c: Clarify when irq processing begins.

Secondary cpus start with local interrupts disabled.
start_secondary() first initializes the new cpu, then it enables the
local interrupts. (although interrupts are enabled within smp_callin()
as well).

Right now, the local interrupts are enabled as a side effect of calling
ipi_call_lock_irq().

The attached patch clarifies when local interrupts are enabled.

Signed-off-by: Manfred Spraul <manfred@colorfullife.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>