ipvlan: properly track tx_errors

ipvlan: properly track tx_errors

Both ipvlan_process_v4_outbound() and ipvlan_process_v6_outbound()
increment dev->stats.tx_errors in case of errors.

Unfortunately there are two issues :

1) ipvlan_get_stats64() does not propagate dev->stats.tx_errors to user.

2) Increments are not atomic. KCSAN would complain eventually.

Use DEV_STATS_INC() to not miss an update, and change ipvlan_get_stats64()
to copy the value back to user.

Fixes: 2ad7bf363841 ("ipvlan: Initial check-in of the IPVLAN driver.")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Mahesh Bandewar <maheshb@google.com>
Link: https://lore.kernel.org/r/20231026131446.3933175-1-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>