KVM: s390: fix cmma migration for multiple memory slots

When multiple memory slots are present the cmma migration code
does not allocate enough memory for the bitmap. The memory slots
are sorted in reverse order, so we must use gfn and size of
slot[0] instead of the last one.

Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
Reviewed-by: Claudio Imbrenda <imbrenda@linux.vnet.ibm.com>
Cc: stable@vger.kernel.org # 4.13+
Fixes: 190df4a2

 (KVM: s390: CMMA tracking, ESSA emulation, migration mode)
Reviewed-by: Cornelia Huck <cohuck@redhat.com>