xen/gntdev: use mmu_interval_notifier_insert

gntdev simply wants to monitor a specific VMA for any notifier events,
this can be done straightforwardly using mmu_interval_notifier_insert()
over the VMA's VA range.

The notifier should be attached until the original VMA is destroyed.

It is unclear if any of this is even sane, but at least a lot of duplicate
code is removed.

Link: https://lore.kernel.org/r/20191112202231.3856-15-jgg@ziepe.ca
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>