coredump filter: fix stack overflow with =all

We translate 'all' to UNIT64_MAX, which has a lot more 'f's. Use the
helper macro, since a decimal uint64_t will always be >> than a hex
representation.

root@image:~# systemd-run -t --property CoredumpFilter=all ls /tmp
Running as unit: run-u13.service
Press ^] three times within 1s to disconnect TTY.
*** stack smashing detected ***: terminated
[137256.320511] systemd[1]: run-u13.service: Main process exited, code=dumped, status=6/ABRT
[137256.320850] systemd[1]: run-u13.service: Failed with result 'core-dump'.

src/shared/coredump-util.c

@@ -158,9 +158,9 @@ int parse_auxv(int log_level,
 }
 
 int set_coredump_filter(uint64_t value) {
-        char t[STRLEN("0xFFFFFFFF")];
+        char t[HEXADECIMAL_STR_MAX(uint64_t)];
 
-        sprintf(t, "0x%"PRIx64, value);
+        xsprintf(t, "0x%"PRIx64, value);
 
         return write_string_file("/proc/self/coredump_filter", t,
                                  WRITE_STRING_FILE_VERIFY_ON_FAILURE|WRITE_STRING_FILE_DISABLE_BUFFER);