net: ethernet: use ip_hdrlen() instead of bit shift

`ip_hdr(skb)->ihl << 2` is the same as `ip_hdrlen(skb)`
Therefore, we should use a well-defined function not a bit shift
to find the header length.

It also compresses two lines to a single line.

Signed-off-by: Moon Yeounsu <yyyynoom@gmail.com>
Reviewed-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: David S. Miller <davem@davemloft.net>