This fixes errors with utilities (such as LVM's vgscan) that try to scan 
all devices. Previously this would generate read errors when 
uninitialized nbd devices were scanned:

# vgscan
   Reading all physical volumes.  This may take a while...
   /dev/nbd0: read failed after 0 of 1024 at 0: Input/output error
   /dev/nbd0: read failed after 0 of 1024 at 509804544: Input/output error
   /dev/nbd0: read failed after 0 of 2048 at 0: Input/output error
   /dev/nbd1: read failed after 0 of 1024 at 509804544: Input/output error
   /dev/nbd1: read failed after 0 of 2048 at 0: Input/output error


 From now on, uninitialized nbd devices will have size zero, which 
prevents these errors.

Patch applies and was tested against 2.6.23-rc2-mm2.

Thanks,
Paul