Blackfin: SMP: disable preempt with smp_processor_id to send messages

The smp_processor_id() API requires that preempt be disabled when calling
it, so make sure it is when we go to send messages to other processors.

Signed-off-by: Sonic Zhang <sonic.zhang@analog.com>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>