net: IPv6: Fix source address for mesh multicast destination

Currently there is chosen the link local address as a source address
for each multicast destination address. It is a bug for OpenThread
network where the mesh-local EID addres should be picked in this case.
This commit fixes it by distinquish the mesh local multicast among any
others.

Signed-off-by: Lukasz Maciejonczyk <lukasz.maciejonczyk@nordicsemi.no>