net: Align IFLA_STATS64 attributes properly on architectures that need it.

Since the nlattr header is 4 bytes in size, it can cause the netlink
attribute payload to not be 8-byte aligned.

This is particularly troublesome for IFLA_STATS64 which contains 64-bit
statistic values.

Solve this by creating a dummy IFLA_PAD attribute which has a payload
which is zero bytes in size.  When HAVE_EFFICIENT_UNALIGNED_ACCESS is
false, we insert an IFLA_PAD attribute into the netlink response when
necessary such that the IFLA_STATS64 payload will be properly aligned.

With help and suggestions from Eric Dumazet.

Signed-off-by: David S. Miller <davem@davemloft.net>