Fix skb truesize underestimations of this driver.

Each frag truesize is exactly rx_frag_size bytes. (2048 bytes per
default)

A driver should not use "sizeof(struct sk_buff)" at all.

Signed-off-by: Eric Dumazet <eric.dumazet>
Signed-off-by: NEric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>