bgpd: Add peer action for PEER_FLAG_IFPEER_V6ONLY flag

bgpd: Add peer action for PEER_FLAG_IFPEER_V6ONLY flag

peer_flag_modify() will always return BGP_ERR_INVALID_FLAG because
the action was not defined for PEER_FLAG_IFPEER_V6ONLY flag.

```
global PEER_FLAG_IFPEER_V6ONLY = 16384;
global BGP_ERR_INVALID_FLAG = -2;

probe process("/usr/lib/frr/bgpd").statement("peer_flag_modify@/root/frr/bgpd/bgpd.c:3975")
{
	if ($flag == PEER_FLAG_IFPEER_V6ONLY && $action->type == 0)
		printf("action not found for the flag PEER_FLAG_IFPEER_V6ONLY\n");
}

probe process("/usr/lib/frr/bgpd").function("peer_flag_modify").return
{
	if ($return == BGP_ERR_INVALID_FLAG)
		printf("return BGP_ERR_INVALID_FLAG\n");
}
```
produces:
action not found for the flag PEER_FLAG_IFPEER_V6ONLY
return BGP_ERR_INVALID_FLAG

$ vtysh -c 'conf t' -c 'router bgp 20' -c 'neighbor eth1 interface v6only remote-as external'

Signed-off-by: Donatas Abraitis <donatas.abraitis@gmail.com>