s390/vmem: rename vmem_add_mem() to vmem_add_range()

Let's match the name to vmem_remove_range().

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: <20200722094558.9828-2-david@redhat.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>