nbd: trivial cleanups

Trivial cleanups for nbd: only the return -EIO one really changes code,
and I've verified all the callers (plus 0 == success, 1 == error
convention is really ugly).

Signed-off-by: Pavel Machek <pavel@suse.cz>
Acked-by: Paul Clements <paul.clements@steeleye.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>