[PATCH] drivers/block/nbd.c: don't defer compile error to runtime

If we can detect a problem at compile time, the compilation should fail.

Signed-off-by: Adrian Bunk <bunk@stusta.de>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>