net/mlx5: Allow creating autogroups with reserved entries

Exclude the last n entries for an autogrouped flow table.

Reserving entries at the end of the FT will ensure that this FG will be
the last to be evaluated. This will be used in the next patch to create
a miss group enabling custom actions on FT miss.

Signed-off-by: Paul Blakey <paulb@mellanox.com>
Reviewed-by: Roi Dayan <roid@mellanox.com>
Reviewed-by: Oz Shlomo <ozsh@mellanox.com>
Reviewed-by: Mark Bloch <markb@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>