hv_netvsc: change member name of struct netvsc_stats

Currently the struct netvsc_stats has a member s_sync
of type u64_stats_sync.
This definition will break kernel build as the macro
netdev_alloc_pcpu_stats requires this member name to be syncp.
(see netdev_alloc_pcpu_stats definition in ./include/linux/netdevice.h)

This patch changes netvsc_stats's member name from s_sync to syncp to fix
the build break.

Signed-off-by: Simon Xiao <sixiao@microsoft.com>
Signed-off-by: David S. Miller <davem@davemloft.net>