When a directory is modified while READDIRPLUS is running,
the file type may change between readdir and lookup.

This patch adds the function FixMode() which allows to
overwrite the file type of the last serialized dirent.

Add test

Fixes https://github.com/hanwen/go-fuse/issues/323

Change-Id: Id45a9c4633e9abcb075588dc610ce8341d104825