datapath: Use u64_stats_sync for datapath and vport stats.

datapath: Use u64_stats_sync for datapath and vport stats.

We currently use a seqcount to prevent reading partial 64-bit stats
on 32-bit CPUs.  u64_stats_sync uses the same logic but elides it on
64-bit and uniprocessor machines.  This improves performance (primarily
on non-x86 architectures) at the cost of not guaranteeing that packet
and byte counts were necessarily read together.

Suggested-by: Stephen Hemminger <shemminger@vyatta.com>
Signed-off-by: Jesse Gross <jesse@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>