vfio/spapr_tce: use mmgrab

vfio/spapr_tce: use mmgrab

Mmgrab was introduced in commit f1f1007644ff ("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>