IB/mlx5: Support flow counters offset for bulk counters

Add support for flow steering counters action with a non-base counter
ID (offset) for bulk counters.

When creating a flow counter object, save the bulk value.  This value is
used when a flow action with a non-base counter ID is requested - to
validate that the required offset is in the range of the allocated bulk.

Link: https://lore.kernel.org/r/20191103140723.77411-1-leon@kernel.org
Signed-off-by: Yevgeny Kliteynik <kliteyn@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Reviewed-by: Mark Bloch <markb@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>