Don't send FBT_DESTROY if the FUSE connection is still PENDING. Also

Don't send FBT_DESTROY if the FUSE connection is still PENDING. Also
don't attempt to determine the size of the root inode. This is because
we don't need to know the size of directories and FBT_GETATTR will also
cause a deadlock if fuse_unmount(3) is called before the file system
has a chance to process fbufs.

Add corresponding regression test.

ok mpi@