KVM: s390: Add explicit padding to struct kvm_s390_keyop

The newly added structure causes a warning about implied padding:

./usr/include/linux/kvm.h:1247:1: error: padding struct size to alignment boundary with 6 bytes [-Werror=padded]

The padding can lead to leaking kernel data and ABI incompatibilies
when used on x86. Neither of these is a problem in this specific
patch, but it's best to avoid it and use explicit padding fields
in general.

Fixes: 0ee4ddc1647b ("KVM: s390: Storage key manipulation IOCTL")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
Signed-off-by: Claudio Imbrenda <imbrenda@linux.ibm.com>