vsprintf: use WARN_ON_ONCE

Signed-off-by: Marcin Slusarz <marcin.slusarz@gmail.com>
Reviewed-by: Frederic Weisbecker <fweisbec@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>