[PATCH] drivers/block/nbd.c compile fix

The Network Block Device driver doesn't compile if NDEBUG is defined.

Signed-off-by: Ingo van Lil <inguin@gmx.de>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>