ovl: fix random return value on mount

commit 8fc646b44385ff0a9853f6590497e43049eeb311 upstream.

On failure to prepare_creds(), mount fails with a random
return value, as err was last set to an integer cast of
a valid lower mnt pointer or set to 0 if inodes index feature
is enabled.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Fixes: 3fe6e52f ("ovl: override creds with the ones from ...")
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>