net: eth_type_trans() should use skb_header_pointer()

eth_type_trans() can read uninitialized memory as drivers
do not necessarily pull more than 14 bytes in skb->head before
calling it.

As David suggested, we can use skb_header_pointer() to
fix this without breaking some drivers that might not expect
eth_type_trans() pulling 2 additional bytes.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Ben Hutchings <bhutchings@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>