openvswitch: rename ->sync to ->syncp

Openvswitch defines u64_stats_sync as ->sync rather than ->syncp,
so fails to compile with netdev_alloc_pcpu_stats(). So just rename it to ->syncp.

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Fixes: 1c213bd2

 (net: introduce netdev_alloc_pcpu_stats() for drivers)
Cc: David S. Miller <davem@davemloft.net>
Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>
Reviewed-by: Flavio Leitner <fbl@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>