net/mlx5e: Copy all L2 headers into inline segment

ConnectX4-Lx uses an inline wqe mode that currently defaults to
requiring the entire L2 header be included in the wqe.
This patch fixes mlx5e_get_inline_hdr_size() to account for
all L2 headers (VLAN, QinQ, etc) using skb_network_offset(skb).

Fixes: e586b3b0 ("net/mlx5: Ethernet Datapath files")
Signed-off-by: Matthew Finlay <matt@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>