net/mlx4_en: fix tx_dropped bug

1) mlx4_en_xmit() can increment priv->stats.tx_dropped, but this variable
is overwritten in mlx4_en_DUMP_ETH_STATS().

2) This increment was not SMP safe, as a port might have many TX queues.

Add a per TX ring tx_dropped to fix these issues.

This is u32 as mlx4_en_DUMP_ETH_STATS() will add a 32bit field.

So lets avoid bugs with SNMP agents having to cope with partial
overwraps. (One of these agents being bond_fold_stats())

Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: Willem de Bruijn <willemb@google.com>
Cc: Eugenia Emantayev <eugenia@mellanox.com>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>