mwan3: Ping -I <interface>: ping: sendto: Permission denied on IPv6

Maintainer: @feckert
Environment: 19.07.07 x86_64
Description: When using interface with ping -I ping gets an EPERM, but works with an IP address.

# ping -I pppoe-wan1 2001:4860:4860::8844
PING 2001:4860:4860::8844 (2001:4860:4860::8844): 56 data bytes
ping: sendto: Permission denied

# ping -I 2607:abcd:8006::2fe7 2001:4860:4860::8844
PING 2001:4860:4860::8844 (2001:4860:4860::8844) from 2607:abcd:8006::2fe7: 56 data bytes
64 bytes from 2001:4860:4860::8844: seq=0 ttl=118 time=12.070 ms
64 bytes from 2001:4860:4860::8844: seq=1 ttl=118 time=12.113 ms
64 bytes from 2001:4860:4860::8844: seq=2 ttl=118 time=11.783 ms
64 bytes from 2001:4860:4860::8844: seq=3 ttl=118 time=11.662 ms
64 bytes from 2001:4860:4860::8844: seq=4 ttl=118 time=12.152 ms
^C
--- 2001:4860:4860::8844 ping statistics ---
5 packets transmitted, 5 packets received, 0% packet loss
round-trip min/avg/max = 11.662/11.956/12.152 ms

And I verified it's using the correct interface in the latter case.