hfs: fix hfs_readdir()

I was looking through static analysis warnings and there is a bug here
that goes all the way back to the start of git.  Basically we're copying
the pointer and nearby garbage instead of the data the fd.key pointer is
pointing to.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Vyacheslav Dubeyko <slava@dubeyko.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>