fix regression from "vsprintf: unify the format decoding layer for its 3 users"

Jeremy Fitzhardinge reported:

> Change fef20d9c

, "vsprintf:
> unify the format decoding layer for its 3 users", causes a
> regression in xenbus which results in no devices getting
> attached to a new domain.

%.*s is broken - fix it.

Reported-by: Jeremy Fitzhardinge <jeremy@goop.org>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>