s390/cert_store: fix string length handling

Building cert_store.o with W=1 reveals this bug:

        CC      arch/s390/kernel/cert_store.o
          arch/s390/kernel/cert_store.c:443:45: warning: ‘sprintf’ may write a terminating nul past the end of the destination [-Wformat-overflow=]
            443 |         sprintf(desc + name_len, ":%04u:%08u", vce->vce_hdr.vc_index, cs_token);
                |                                             ^
          arch/s390/kernel/cert_store.c:443:9: note: ‘sprintf’ output between 15 and 18 bytes into a destination of size 15
            443 |         sprintf(desc + name_len, ":%04u:%08u", vce->vce_hdr.vc_index, cs_token);

Fix this by using the correct maximum width for each integer component
in both buffer length calculation and format string. Also switch to
using snprintf() to guard against potential future changes to the
integer range of each component.

Fixes: 8cf57d72 ("s390: add support for user-defined certificates")
Reported-by: Heiko Carstens <hca@linux.ibm.com>
Reviewed-by: Alexander Gordeev <agordeev@linux.ibm.com>
Signed-off-by: Peter Oberparleiter <oberpar@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>