vfio/spapr_tce: 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>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>

drivers/vfio/vfio_iommu_spapr_tce.c

@@ -79,7 +79,7 @@ static long tce_iommu_mm_set(struct tce_container *container)
 	}
 	BUG_ON(!current->mm);
 	container->mm = current->mm;
-	atomic_inc(&container->mm->mm_count);
+	mmgrab(container->mm);
 
 	return 0;
 }