KVM: s390: Replace sprintf with snprintf for buffer safety

KVM: s390: Replace sprintf with snprintf for buffer safety

Replace sprintf() with snprintf() when formatting debug names to prevent
potential buffer overflow. The debug_name buffer is 16 bytes, and while
unlikely to overflow with current PIDs, using snprintf() provides proper
bounds checking.

Signed-off-by: Josephine Pfeiffer <hi@josie.lol>
[frankja@linux.ibm.com: Fixed subject prefix]
Signed-off-by: Janosch Frank <frankja@linux.ibm.com>