batman-adv: Demote batadv-on-batadv skip error message

The error message "Cannot find parent device" was shown for users of
macvtap (on batadv devices) whenever the macvtap was moved to a different
netns. This happens because macvtap doesn't provide an implementation for
rtnl_link_ops->get_link_net.

The situation for which this message is printed is actually not an error
but just a warning that the optional sanity check was skipped. So demote
the message from error to warning and adjust the text to better explain
what happened.

Reported-by: Leonardo Mörlein <freifunk@irrelefant.net>
Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>