mwan3: use network_get_preferred_ipaddr6

mwan3: use network_get_preferred_ipaddr6

This updates mwan3 to use network_get_preferred_ipaddr6 instead of
network_get_ipaddr6 if possible to determine a source ip for the
connectivity checks. This avoids issues where the first ip address
that is returned from network_get_ipaddr6 does not work anymore while
the preferred one returned from network_get_preferred_ipaddr6 works.

Signed-off-by: Jonas Lochmann <git@inkompetenz.org>