Use struct stat for storing attributes in fusebufs, because using struct

Use struct stat for storing attributes in fusebufs, because using struct
vattr in userspace is suboptimal as some related helpers are not
available, e.g. VATTR_NULL() and IFTOVT(). The conversion is now done in
the kernel where it belongs. As a side effect the <sys/vnode.h> include
can be removed from libfuse.

tweaks and ok guenther