cmfs-tools: add missing trunate_log entry in system_files[]

In mkfs.c, system_files[] array does not have truncate_log entry, while
in include/cmfs-kernel/cmfs_fs.h, TRUNCATE_LOG_SYSTEM_INODE is defined
in system file index enum.

In this case, there will be a segment faul occures in mkfs.c when
initializing all system inodes.

This commit fix this by,
1) add truncate_log entry in system_files[]
2) in main.c, when calling sprintf, if system_files[i].name does not
have format string, do not print num postfix into fname. (this is not
very ncessary, because sprintf will not scan the third parameter if no
format string in system_files[i].name, this change just make the logic
more clear IMHO).