[XFRM]: Rearrange struct xfrm_aevent_id for better compatibility.

struct xfrm_aevent_id needs to be 32-bit + 64-bit align friendly.

Based upon suggestions from Yoshifuji.

Signed-off-by: Jamal Hadi Salim <hadi@cyberus.ca>
Signed-off-by: David S. Miller <davem@davemloft.net>