Since snprintf() returns the would-be-output size instead of the
actual output size, the succeeding calls may go beyond the given
buffer limit.  Fix it by replacing with scnprintf().
Reviewed-by: NChristoph Hellwig <hch@lst.de>
Signed-off-by: NTakashi Iwai <tiwai@suse.de>
Signed-off-by: NKeith Busch <kbusch@kernel.org>