vfio: vfio_pci_nvlink2: 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/pci/vfio_pci_nvlink2.c

@@ -161,7 +161,7 @@ static int vfio_pci_nvgpu_mmap(struct vfio_pci_device *vdev,
 	data->useraddr = vma->vm_start;
 	data->mm = current->mm;
 
-	atomic_inc(&data->mm->mm_count);
+	mmgrab(data->mm);
 	ret = (int) mm_iommu_newdev(data->mm, data->useraddr,
 			vma_pages(vma), data->gpu_hpa, &data->mem);