vrrp: IPv6 mcast src_addr handling and VMAC fix

Properly bind socket for v6 use-case when mcasr_src_ip is in use or when
VMAC is used. This patch fix VRRP VMAC in native_ipv6 mode, previous code
just use the vmac interface link-local IP Address as src_ip leading to a
corner case (to keep polite).