s390/vmemmap: extend modify_pagetable() to handle vmemmap

Extend our shiny new modify_pagetable() to handle !direct (vmemmap)
mappings. Convert vmemmap_populate() and implement vmemmap_free().

Cc: Vasily Gorbik <gor@linux.ibm.com>
Cc: Christian Borntraeger <borntraeger@de.ibm.com>
Cc: Gerald Schaefer <gerald.schaefer@de.ibm.com>
Signed-off-by: David Hildenbrand <david@redhat.com>
Message-Id: <20200722094558.9828-4-david@redhat.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>