Don't create LFNs for . and ..

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

    LFN for .
    .
    LFN for ..
    ..

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

src/dir.rs

@@ -530,6 +530,12 @@ impl<'a, IO: ReadWriteSeek, TP: TimeProvider, OCC: OemCpConverter> Dir<'a, IO, T
         Ok((stream, start_pos))
     }
 
+    fn alloc_sfn_entry(&self) -> Result<(DirRawStream<'a, IO, TP, OCC>, u64), Error<IO::Error>> {
+        let mut stream = self.find_free_entries(1)?;
+        let start_pos = stream.seek(io::SeekFrom::Current(0))?;
+        Ok((stream, start_pos))
+    }
+
     fn write_entry(
         &self,
         name: &str,
@@ -540,8 +546,13 @@ impl<'a, IO: ReadWriteSeek, TP: TimeProvider, OCC: OemCpConverter> Dir<'a, IO, T
         validate_long_name(name)?;
         // convert long name to UTF-16
         let lfn_utf16 = Self::encode_lfn_utf16(name);
-        // write LFN entries
-        let (mut stream, start_pos) = self.alloc_and_write_lfn_entries(&lfn_utf16, raw_entry.name())?;
+        // write LFN entries, except for . and .., which need to be at
+        // the first two slots and don't need LFNs anyway
+        let (mut stream, start_pos) = if name == "." || name == ".." {
+            self.alloc_sfn_entry()?
+        } else {
+            self.alloc_and_write_lfn_entries(&lfn_utf16, raw_entry.name())?
+        };
         // write short name entry
         raw_entry.serialize(&mut stream)?;
         // Get position directory stream after entries were written