net/mlx5e: Add headroom only to the first fragment in legacy RQ

Currently, rq->buff.headroom is applied to all fragments in legacy RQ.
In the linear mode, there is a non-zero headroom, but there is only one
fragment per packet. In the non-linear mode, the headroom is zero.

This commit changes the logic to apply the headroom only to the first
fragment. The current behavior remains the same for both linear and
non-linear modes. However, it allows the next commit to enable headroom
for the non-linear mode, which will be applied only to the first
fragment.

Signed-off-by: Maxim Mikityanskiy <maximmi@nvidia.com>
Reviewed-by: Tariq Toukan <tariqt@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>