IB/mlx4: Add interface for selecting VFs to enable QP0 via MLX proxy QPs

This commit adds the sysfs interface for enabling QP0 on VFs for
selected VF/port.

By default, no VFs are enabled for QP0 operation.

To enable QP0 operation on a VF/port, under
/sys/class/infiniband/mlx4_x/iov/<b:d:f>/ports/x there are two new entries:

- smi_enabled (read-only). Indicates whether smi is currently
  enabled for the indicated VF/port

- enable_smi_admin (rw). Used by the admin to request that smi
  capability be enabled or disabled for the indicated VF/port.
  0 = disable, 1 = enable.
  The requested enablement will occur at the next reset of the
  VF (e.g. driver restart on the VM which owns the VF).

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