eth: Pull header from first fragment via eth_get_headlen

We want to try and pull the L4 header in if it is available in the first
fragment.  As such add the flag to indicate we want to pull the headers on
the first fragment in.

Signed-off-by: Alexander Duyck <aduyck@mirantis.com>
Acked-by: Tom Herbert <tom@herbertland.com>
Signed-off-by: David S. Miller <davem@davemloft.net>