mlx4: make validate_loopback() more generic

Testing a boolean in fast path is not worth duplicating
the code allocating packets, when GRO is on or off.

If this proves to be a problem, we might later use a jump label.

Next patch will remove this duplicated code and ease code review.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Acked-by: Tariq Toukan <tariqt@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>