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().

Link: https://lore.kernel.org/r/20200320084429.1803-1-tiwai@suse.deSigned-off-by: Takashi Iwai <tiwai@suse.de>