IB/mlx4: Add IB counters table

This is an infrastructure step for allocating and attaching more than
one counter to QPs on the same port. Allocate a counters table and
manage the insertion and removals of the counters in load and unload of
mlx4 IB.

Signed-off-by: Eran Ben Elisha <eranbe@mellanox.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>