mlx4_core: reserve UARs for userspace consumers

Do not allow a kernel consumer to allocate a UAR to serve for blue flame if the
number of available UARs gets below MLX4_NUM_RESERVED_UARS (currently 8). This
will allow userspace apps to open a device file and run things like
ibv_devinfo.

Signed-off-by: Eli Cohen <eli@mellanox.co.il>
Signed-off-by: David S. Miller <davem@davemloft.net>