ovl: set lower layer st_dev only if setting lower st_ino

ovl: set lower layer st_dev only if setting lower st_ino

BugLink: http://bugs.launchpad.net/bugs/1769723
commit 9f99e50d460ac7fd5f6c9b97aad0088c28c8656d upstream.

For broken hardlinks, we do not return lower st_ino, so we should
also not return lower pseudo st_dev.

Fixes: a0c5ad307ac0 ("ovl: relax same fs constraint for constant st_ino")
Cc: <stable@vger.kernel.org> #v4.15
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>