s390/diag: use __packed __aligned

Use __packed __aligned instead of __attribute__((packed, aligned(X)));
to match the rest of the file.

Signed-off-by: Sven Schnelle <svens@linux.ibm.com>
Acked-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>