Don't create LFNs for . and ..

[badicsalex]

The current code makes invalid dir entries, because the first two slots should be . and .., but with LFNs, it's

1. LFN for .
   

2. .
   

3. LFN for ..
   

4. ..
   

We should probably not create LFN's when not needed anyway, but that's not implemented in this patch.

fsck output for the current code:

fsck from util-linux 2.40.2
fsck.fat 4.2 (2021-01-31)
/home/A.
  Expected a valid '.' entry in this slot.
  Moving entry down.
/home/.
  Expected a valid '..' entry in this slot.
  Moving entry down.
/home/..
  Bad short file name (..).
  Auto-renaming it.
  Renamed to FSCK0000.000
/home/.
  Bad short file name (.).
  Auto-renaming it.
  Renamed to FSCK0000.001
/  and
/home/FSCK0000.000
  share clusters.
  Truncating second to 0 bytes because first is FAT32 root dir.
/home/FSCK0000.001
 Start does point to containing directory. Deleting entry.

[rafalh]

Thanks for this change! It would be great to run fsck in CI to catch such issues automatically.
Could you please add CHANGELOG entry in "Bug fixes:" section?

[badicsalex]

I put this patch together with #99 for easier merging.