net/mlx5: SF, Improve performance in SF allocation

Avoid second traversal on the SF table by recording the first free entry
and using it in case the looked up entry was not found in the table.

Signed-off-by: Eli Cohen <elic@nvidia.com>
Signed-off-by: Parav Pandit <parav@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>