As pointed out by @slackner at
https://github.com/hanwen/go-fuse/commit/8581b7e950b2d520370eba8997ddd9493192e9c7#r35845728
FixMode() failed to take into account the length
of the file name and the padding.

To fix this without adding a lot of additional
pointer arithmetic, add the "lastDirent" field
to DirEntryList. This stores the last serialized
_Dirent, so _Dirent.Typ can be accessed easily
by FixMode().

Change-Id: I6ca3fe551e1a072779b1d5ed5293add057e55287