s390/vmem: consolidate vmem_add_range() and vmem_remove_range()

We want to have only a single pagetable walker and reuse the same
functionality for vmemmap handling. Let's start by consolidating
vmem_add_range() and vmem_remove_range(), converting it into a
recursive implementation.

A recursive implementation makes it easier to expand individual cases
without harming readability. In addition, we minimize traversing the
whole hierarchy over and over again.

One change is that we don't unmap large PMDs/PUDs when not completely
covered by the request, something that should never happen with direct
mappings, unless one would be removing in other granularity than added,
which would be broken already.

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: <20200722094558.9828-3-david@redhat.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>