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

Linux upstream commit: 79cdb0aaea8b5478db34afa1d4d5ecc808689a67
Signed-off-by: Mark Bloch <mbloch@nvidia.com>
Sponsored by: NVidia networking
MFC after: 1 week