Let's cleanup empty page tables. Consider only page tables that fully
fall into the idendity mapping and the vmemmap range.

As there are no valid accesses to vmem/vmemmap within non-populated ranges,
the single tlb flush at the end should be sufficient.

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: <20200722094558.9828-7-david@redhat.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>