xen/gntdev: Do not use mm notifiers with autotranslating guests

Commit d3eeb1d7 ("xen/gntdev: use mmu_interval_notifier_insert")
missed a test for use_ptemod when calling mmu_interval_read_begin(). Fix
that.

Fixes: d3eeb1d7 ("xen/gntdev: use mmu_interval_notifier_insert")
CC: stable@vger.kernel.org # 5.5
Reported-by: Ilpo Järvinen <ilpo.jarvinen@cs.helsinki.fi>
Tested-by: Ilpo Järvinen <ilpo.jarvinen@cs.helsinki.fi>
Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Reviewed-by: Jason Gunthorpe <jgg@mellanox.com>
Acked-by: Juergen Gross <jgross@suse.com>