bonding: Fix extraction of ports from the packet headers

Wrong hash sends single stream to multiple output interfaces.

The offset calculation was relative to skb->head, fix it to be relative
to skb->data.

Fixes: a815bde5

 ("net, bonding: Refactor bond_xmit_hash for use with
xdp_buff")
Reviewed-by: Jussi Maki <joamaki@gmail.com>
Reviewed-by: Saeed Mahameed <saeedm@nvidia.com>
Reviewed-by: Gal Pressman <gal@nvidia.com>
Signed-off-by: Moshe Tal <moshet@nvidia.com>
Acked-by: Jay Vosburgh <jay.vosburgh@canonical.com>
Signed-off-by: David S. Miller <davem@davemloft.net>