xen/gntdev: correctly unmap unlinked maps in mmu notifier

If gntdev_ioctl_unmap_grant_ref is called on a range before unmapping
it, the entry is removed from priv->maps and the later call to
mn_invl_range_start won't find it to do the unmapping. Fix this by
creating another list of freeable maps that the mmu notifier can search
and use to unmap grants.

Signed-off-by: Daniel De Graaf <dgdegra@tycho.nsa.gov>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>