This pull request contains some NBD cleanups.
The documentation was updated to be extensible for future features and the
module parameters were added.
NBD had an internal header located at include/linux/nbd.h which was only used
by nbd.c. So I moved the header content from include/ to the top of nbd.c.
The debugging prints of nbd which contained a basic filter functionality were
replaced by dev_dbg(). This removes the ability to filter by categories. But it
is now possible to use dynamic debugging.
The other patches are some small simplifications.