mlx4_en: Consider napi_get_frags() failure.

If failed to get skb frags using napi_get_frags(),
the packet is dropped.

Reported-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: Yevgeny Petrilin <yevgenyp@mellanox.co.il>
Signed-off-by: David S. Miller <davem@davemloft.net>