datapath: Fix unaligned access when storing stats.

Both datapath and vport stats contain 64-bit members in a struct
but we write them directly in Netlink attributes which only
guarantee 32-bit alignment.  This causes problems on RISC
architectures that care about alignment so this computes the stats
on the stack and then memcpy's them.

Signed-off-by: Jesse Gross <jesse@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>