KVM: s390: Enable KVM_CAP_NR_MEMSLOTS on s390

Return KVM_USER_MEM_SLOTS in kvm_dev_ioctl_check_extension().

Signed-off-by: Nick Wang <jfwang@us.ibm.com>
Reviewed-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Signed-off-by: Gleb Natapov <gleb@redhat.com>