fix readdirplus when filler is called with zero offset (#896)

fix readdirplus when filler is called with zero offset (#896)

fixes #235

In fill_dir_plus(), there's a lookup for caching dirent attributes.
However, when offset = 0 the cache metadata from the lookup is lost
as only the entry attributes are passed when added to the list. Kernel
doesn't cache the attributes since .ino = 0.

This change moves the entry lookup to happen just before the relevant
fuse_add_direntry_plus() calls