s390/zcrypt: replace snprintf/sprintf with scnprintf

snprintf() may not always return the correct size of used bytes but
instead the length the resulting string would be if it would fit into
the buffer. So scnprintf() is the function to use when the real length
of the resulting string is needed.

Replace all occurrences of snprintf() with scnprintf() where the return
code is further processed. Also find and fix some occurrences where
sprintf() was used.

Suggested-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Harald Freudenberger <freude@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>