net/mlx5e: Refactor inline header size calculation in the TX path

As preparation for the next patch, don't increase ihs to calculate
ds_cnt and then decrease it, but rather calculate the intermediate value
temporarily. This code has the same amount of arithmetic operations, but
now allows to split out ds_cnt calculation, which will be performed in
the next patch.
Signed-off-by: Maxim Mikityanskiy <maximmi@mellanox.com>
Reviewed-by: Tariq Toukan <tariqt@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>

drivers/net/ethernet/mellanox/mlx5/core/en_tx.c

@@ -307,9 +307,9 @@ void mlx5e_sq_xmit(struct mlx5e_txqsq *sq, struct sk_buff *skb,
 	ds_cnt += skb_shinfo(skb)->nr_frags;
 
 	if (ihs) {
-		ihs += !!skb_vlan_tag_present(skb) * VLAN_HLEN;
+		u16 inl = ihs + !!skb_vlan_tag_present(skb) * VLAN_HLEN - INL_HDR_START_SZ;
 
-		ds_cnt_inl = DIV_ROUND_UP(ihs - INL_HDR_START_SZ, MLX5_SEND_WQE_DS);
+		ds_cnt_inl = DIV_ROUND_UP(inl, MLX5_SEND_WQE_DS);
 		ds_cnt += ds_cnt_inl;
 	}
 
@@ -348,12 +348,12 @@ void mlx5e_sq_xmit(struct mlx5e_txqsq *sq, struct sk_buff *skb,
 	eseg->mss = mss;
 
 	if (ihs) {
-		eseg->inline_hdr.sz = cpu_to_be16(ihs);
 		if (skb_vlan_tag_present(skb)) {
-			ihs -= VLAN_HLEN;
+			eseg->inline_hdr.sz = cpu_to_be16(ihs + VLAN_HLEN);
 			mlx5e_insert_vlan(eseg->inline_hdr.start, skb, ihs);
 			stats->added_vlan_packets++;
 		} else {
+			eseg->inline_hdr.sz = cpu_to_be16(ihs);
 			memcpy(eseg->inline_hdr.start, skb->data, ihs);
 		}
 		dseg += ds_cnt_inl;