openrisc: use mmgrab

Mmgrab was introduced in commit f1f10076 ("mm: add new mmgrab()
helper") and most of the kernel was updated to use it. Update a
remaining file.

The semantic patch that makes this change is as follows:
(http://coccinelle.lip6.fr/)

<smpl>
@@ expression e; @@
- atomic_inc(&e->mm_count);
+ mmgrab(e);
</smpl>
Signed-off-by: Julia Lawall <Julia.Lawall@inria.fr>
Signed-off-by: Stafford Horne <shorne@gmail.com>
[shorne: Added missing sched/mm.h include]

arch/openrisc/kernel/smp.c

@@ -14,6 +14,7 @@
 #include <linux/smp.h>
 #include <linux/cpu.h>
 #include <linux/sched.h>
+#include <linux/sched/mm.h>
 #include <linux/irq.h>
 #include <asm/cpuinfo.h>
 #include <asm/mmu_context.h>
@@ -113,7 +114,7 @@ asmlinkage __init void secondary_start_kernel(void)
 	 * All kernel threads share the same mm context; grab a
 	 * reference and switch to it.
 	 */
-	atomic_inc(&mm->mm_count);
+	mmgrab(mm);
 	current->active_mm = mm;
 	cpumask_set_cpu(cpu, mm_cpumask(mm));