mlx4: only pull headers into skb head

Use the new fancy eth_get_headlen() to pull exactly the headers
into skb->head.

This speeds up GRE traffic (or more generally tunneled traffuc),
as GRO can aggregate up to 17 MSS per GRO packet instead of 8.

(Pulling too much data was forcing GRO to keep 2 frags per MSS)

Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Amir Vadai <amirv@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>