Make command structure sizes the same on 32-bit and 64-bit

Make command structure sizes the same on 32-bit and 64-bit

Add 4-byte reserved members to the new destroy CQ, destroy QP and
destroy SRQ command structures so that they become a multiple of 8
bytes in size.  This fixes the structures so they have the same size
on both 32-bit and 64-bit architectures (which is required so that
32-bit userspace on a 64-bit kernel works correctly).

Signed-off-by: Roland Dreier <rolandd@cisco.com>