metag: SMP: Fix 4KiB stack setup on secondary CPUs

Back in early 2008, 4KiB stack support was added to reduce memory
consumption on workloads with lots of threads, using a separate IRQ
stack per CPU to alleviate stack pressure. However the SMP code added a
year and a half later didn't set up the IRQ stack when bringing up
secondary CPUs, resulting in a crash when SMP is configured with 4KiB
stacks, as soon as the first interrupt arrived on a secondary CPU.

Fix with calls to irq_ctx_exit()/irq_ctx_exit() when bringing up/down a
CPU.

Signed-off-by: James Hogan <james.hogan@imgtec.com>
Cc: linux-metag@vger.kernel.org

arch/metag/kernel/smp.c

@@ -312,6 +312,7 @@ void cpu_die(void)
 {
 	local_irq_disable();
 	idle_task_exit();
+	irq_ctx_exit(smp_processor_id());
 
 	(void)cpu_report_death();
 
@@ -366,6 +367,7 @@ asmlinkage void secondary_start_kernel(void)
 		panic("No TBI found!");
 
 	per_cpu_trap_init(cpu);
+	irq_ctx_init(cpu);
 
 	preempt_disable();