vxlan: Use helper functions to update stats

Use the helper functions dev_sw_netstats_rx_add() and
dev_sw_netstats_tx_add() to update stats, which helps to
provide code readability.

Signed-off-by: Li Zetao <lizetao1@huawei.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Nikolay Aleksandrov <razor@blackwall.org>
Signed-off-by: David S. Miller <davem@davemloft.net>