This patch allows a Network Block Device to be mounted locally 
(nbd-client to nbd-server over 127.0.0.1).

It creates a kthread to avoid the deadlock described in NBD tools 
documentation. So, if nbd-client hangs waiting for pages, the kblockd 
thread can continue its work and free pages.

I have tested the patch to verify that it avoids the hang that occurs 
when writing to a localhost nbd connection. I have also tested to verify 
that no performance degradation results from the additional thread and 
queue.

Patch originally from Laurent Vivier.

Thanks,
Paul