net/mlx5: Move the entry index allocator to flow group

When new flow table entry is added, we search for free index
in the flow group and not in the flow table, therefore we can move
the allocator from flow table to flow group.
In downstream patches it will enable us to lock smaller part
of the steering tree.

Signed-off-by: Maor Gottlieb <maorg@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>