net/mlx4_core: Add sink counter

Reserve the last valid counter index for "sink" counter, when a
new counter cannot be allocated, the driver will use this counter.

In order to avoid allocating this counter on any other flow, fix the
indices bitmap allocation range, and reserve the sink counter index.

Add macro for the sink counter index and replace all appearences of the
index with the macro.

Signed-off-by: Eran Ben Elisha <eranbe@mellanox.com>
Signed-off-by: Hadar Hen Zion <hadarh@mellanox.com>
Signed-off-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>