net/mlx5: Export steering related functions

Export
 mlx5_create_flow_table()
 mlx5_create_flow_group()
 mlx5_destroy_flow_group().

These symbols are required by the VDPA implementation to create rules
that consume VDPA specific traffic.

We do not deal with putting the prototypes in a header file since they
already exist in include/linux/mlx5/fs.h.

Signed-off-by: Eli Cohen <elic@nvidia.com>
Reviewed-by: Mark Bloch <mbloch@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>