mwan3: common.sh: fix src_ip detection for ipv6-PD

mwan3: common.sh: fix src_ip detection for ipv6-PD

IPv6 via PPPoE often receives a prefix, not an address. In that case
mwan3_get_src_ip would not find an address and fall back to ::.  However
in case of multiple IPv6 upstreams, this no longer means a ping (such as
done by uptime check) succeeds, for there are multiple addresses.

With this, mwan3 can effectively load-balance/fail-over on a PPPoE IPv6
prefix and a secondary (LTE) acquired /64 without immediately disabling
the IPv6 prefix interface as soon as the secondary IPv6 interface comes
up.

I think this change is also a fix to #26690
but I'm not 100% sure of that.

Signed-off-by: Fabian Groffen <redacted>