drivers: vxlan: vnifilter: per vni stats

Add per-vni statistics for vni filter mode. Counting Rx/Tx
bytes/packets/drops/errors at the appropriate places.

This patch changes vxlan_vs_find_vni to also return the
vxlan_vni_node in cases where the vni belongs to a vni
filtering vxlan device

Signed-off-by: Nikolay Aleksandrov <nikolay@nvidia.com>
Signed-off-by: Roopa Prabhu <roopa@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>