net/mlx5e: Enhance RX SKB headroom logic

In the RX memory scheme of non Striding RQ, we use linear SKBs.
Keeping NET_IP_ALIGN in headroom can improve performance on some archs.
In addition, take this headroom into account when calculating the
LRO WQE size.

These are not needed in Striding RQ as they're done implicitly
within the non-linear SKB allocation.

Fixes: 1bfecfca

 ("net/mlx5e: Build RX SKB on demand")
Signed-off-by: Tariq Toukan <tariqt@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>