[MIPS] SMTC: Make ack_bad_irq() safe with no IM backstop.

Issue reported and original patch by Kevin Kissel, cleaner (imho)
implementation by me.

Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

arch/mips/kernel/smtc.c

@@ -25,8 +25,11 @@
 #include <asm/smtc_proc.h>
 
 /*
- * This file should be built into the kernel only if CONFIG_MIPS_MT_SMTC is set.
+ * SMTC Kernel needs to manipulate low-level CPU interrupt mask
+ * in do_IRQ. These are passed in setup_irq_smtc() and stored
+ * in this table.
  */
+unsigned long irq_hwmask[NR_IRQS];
 
 #define LOCK_MT_PRA() \
 	local_irq_save(flags); \