parisc/mm: Ensure IRQs are off in switch_mm()

This is because of commit f98db601 ("sched/core: Add switch_mm_irqs_off()
and use it in the scheduler") in which switch_mm_irqs_off() is called by the
scheduler, vs switch_mm() which is used by use_mm().

This patch lets the parisc code mirror the x86 and powerpc code, ie. it
disables interrupts in switch_mm(), and optimises the scheduler case by
defining switch_mm_irqs_off().
Signed-off-by: Helge Deller <deller@gmx.de>
Cc: stable@vger.kernel.org # v4.7+
Signed-off-by: Helge Deller <deller@gmx.de>

arch/parisc/include/asm/mmu_context.h

@@ -49,15 +49,26 @@ static inline void load_context(mm_context_t context)
 	mtctl(__space_to_prot(context), 8);
 }
 
-static inline void switch_mm(struct mm_struct *prev, struct mm_struct *next, struct task_struct *tsk)
+static inline void switch_mm_irqs_off(struct mm_struct *prev,
+		struct mm_struct *next, struct task_struct *tsk)
 {
-
 	if (prev != next) {
 		mtctl(__pa(next->pgd), 25);
 		load_context(next->context);
 	}
 }
 
+static inline void switch_mm(struct mm_struct *prev,
+		struct mm_struct *next, struct task_struct *tsk)
+{
+	unsigned long flags;
+
+	local_irq_save(flags);
+	switch_mm_irqs_off(prev, next, tsk);
+	local_irq_restore(flags);
+}
+#define switch_mm_irqs_off switch_mm_irqs_off
+
 #define deactivate_mm(tsk,mm)	do { } while (0)
 
 static inline void activate_mm(struct mm_struct *prev, struct mm_struct *next)