mlx4_core: Add network flow counters

ConnectX devices support a set of flow counters that can be attached
to a set containing one or more QPs.  Each such counter tracks receive
and transmit packets and bytes of these QPs.  This patch queries the
device to check support for counters, handles initialization of the
HCA to enable counters, and initializes a bitmap allocator to control
counter allocations.  Derived from patch by Eli Cohen <eli@mellanox.co.il>.

Signed-off-by: Or Gerlitz <ogerlitz@mellanox.co.il>
Signed-off-by: Roland Dreier <roland@purestorage.com>