nvme reservation commands during controller reset

[Sagi Grimberg]

>> So what is the semantics we want to provide? a best-effort one? not very
>> different than what we have today. We could, if we choose to, have the
>> pr context to wait for a path_map_changed completion and have
>> nvme_mpath_set_live and nvme_mpath_check_last_path do complete_all on
>> this completion.
>>
>> But the question is what the semantics we want to provide...
> 
> But it might never change..  We could give it the default max_retries
> based retries for any reasons if we want to be fancy.

Is there a difference between sync read and pr ioctl on the mpath device 
node?

Currently the read will block until it absolutely cannot issue the
request, but the ioctl will block until there is no available path.

I think we should have the same semantics for both. Unless convinced
otherwise..