This fourth patch simply removes the blksize_bits field from the 
nbd_device struct and driver implementation. How this field made it into 
this driver to begin with is a mystery (where was Al Viro when that 
patch was submitted??). :-) This patch modifies both drivers/block/nbd.c 
and include/linux/nbd.h files. It's intended to be applied incrementally 
on top of my third patch (for enhanced diagnostics support). I suspect 
this patch is small enough however that it may apply correctly without 
the preceding patches but I have not tried it myself.

There are other things that can be cleaned up this way but this change 
was so very simple I wanted to get it out of the way early on. As 
always, comments welcome.