Fix junk readdirplus results when filesystem not filling stat info

Fix junk readdirplus results when filesystem not filling stat info

Commit dd95d13a ("fix readdirplus when filler is called with zero offset
(#896)) broke readdirplus with passthrough example command:

    passthrough -o auto_cache,modules=subdir,subdir=/src /mnt

The /src directory looks like this:

    ~# ls -l /src
    total 0
    drwx------ 3 root root 60 Jan  2 17:51 testdir

And the fuse directory looks like this:

    ~# ls -l /mnt
    total 0
    d--------- 0 root root 0 Jan  1  1970 testdir

Because readdir_fill_from_list() ignores the fact that filesystem
did not pass the FUSE_FILL_DIR_PLUS flag with valid stat info.

Signed-off-by: Amir Goldstein <amir73il@gmail.com>