[ARM] Solve buggy smp_processor_id() usage

BUG: using smp_processor_id() in preemptible [00000001] code: opcontrol/427

Resolve this bug by ensuring that we're not using smp_processor_id() in
a preemptable context (by disabling preemption.)

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>