net: fix unaligned access in IFLA_STATS64

Tony Luck observes that the original IFLA_STATS64 submission causes
unaligned accesses. This is because nla_data() returns a pointer to a
memory region that is only aligned to 32 bits. Do some memcpying to
workaround this.

Signed-off-by: Jan Engelhardt <jengelh@medozas.de>
Signed-off-by: David S. Miller <davem@davemloft.net>