What is _KERNEL_OPT, and should #includes of opt_*.h files be wrapped in a 
   check for it? I.e.

_KERNEL_OPT is defined by all kernel builds and most opt_*.h files should
be wrapped with it like your first example below.  it is not defined by LKM's
and it useful for them.  you may want to add a way to enable this option in
the vnd LKM (or just enable it always - i wouldn't object and i'm the author
of that LKM.)

   
    	#if defined(_KERNEL_OPT)
    	#include "opt_vnd_with_compression.h"
    	#endif
   
   or
   
    	#include "opt_vnd_with_compression.h"
   ?


.mrg.