ovl: mark parent impure on ovl_link()

When linking a file with copy up origin into a new parent, mark the
new parent dir "impure".

Fixes: ee1d6d37 ("ovl: mark upper dir with type origin entries "impure"")
Cc: <stable@vger.kernel.org> # v4.12
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>