s390/vmemmap: cleanup when vmemmap_populate() fails

Cleanup what we partially added in case vmemmap_populate() fails. For
vmem, this is already handled by vmem_add_mapping().

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: <20200722094558.9828-5-david@redhat.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>