net/mlx5e: Fix inline header size calculation

mlx5e_get_inline_hdr_size didn't take into account the vlan insertion
into the inline WQE segment.
This could lead to max inline violation in cases where
skb_headlen(skb) + VLAN_HLEN >= sq->max_inline.

Fixes: 3ea4891d ("net/mlx5e: Fix LSO vlan insertion")
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>
Signed-off-by: Achiad Shochat <achiad@mellanox.com>
Signed-off-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>