IPoIB: Fix pkey change flow for virtualization environments

IPoIB's required behaviour w.r.t to the pkey used by the device is the following:

- For "parent" interfaces (e.g ib0, ib1, etc) who are created
  automatically as a result of hot-plug events from the IB core, the
  driver needs to take whatever pkey vlaue it finds in index 0, and
  stick to that index.

- For child interfaces (e.g ib0.8001, etc) created by admin directive,
  the driver needs to use and stick to the value provided during its
  creation.

In SR-IOV environment its possible for the VF probe to take place
before the cloud management software provisions the suitable pkey for
the VF in the paravirtualed PKEY table index 0. When this is the case,
the VF IB stack will find in index 0 an invalide pkey, which is all
zeros.

Moreover, the cloud managment can assign the pkey value at index 0 at
any time of the guest life cycle.

The correct behavior for IPoIB to address these requirements for
parent interfaces is to use PKEY_CHANGE event as trigger to optionally
re-init the device pkey value and re-create all the relevant resources
accordingly, if the value of the pkey in index 0 has changed (from
invalid to valid or from valid value X to invalid value Y).

This patch enhances the heavy flushing code which is triggered by pkey
change event, to behave correctly for parent devices. For child
devices, the code remains the same, namely chases pkey value and not
index.

Signed-off-by: Erez Shitrit <erezsh@mellanox.com>
Signed-off-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>