vxlan: Fix racy device stats updates.

VXLAN devices update their stats locklessly. Therefore these counters
should either be stored in per-cpu data structures or the updates
should be done using atomic increments.

Since the net_device_core_stats infrastructure is already used in
vxlan_rcv(), use it for the other rx_dropped and tx_dropped counter
updates. Update the other counters atomically using DEV_STATS_INC().

Fixes: d342894c ("vxlan: virtual extensible lan")
Signed-off-by: Guillaume Nault <gnault@redhat.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>