s390/vmem: cleanup empty page tables

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>