Don't mmap stats files

To avoid getting killed by SIGBUS when a file write fails.
Honestly, I don't think it's worth the trouble. These files are created very rarely (one or two files in the lifetime of each connman service), they are very small, taking only one block in the fs. What's far more likely is that the block occupied by the stats file goes bad at some point. Furthermore, it's pretty much guaranteed to happen at some point. I would very much appreciate if the file system would silently move the file into a fresh working block for me in such case. But I don't know what would really happen in real life and therefore don't know how to correctly handle it (or even if there is anything for me to handle), so I decided not to handle this case at all.