Staging: hv: Fix a bug affecting IPv6

Fix a bug affecting IPv6
Added the multicast flag for proper IPv6 function.

Reported-by: Toshikazu Sakai <toshikas@microsoft.com>
Signed-off-by: Hank Janssen <hjanssen@microsoft.com>
Signed-off-by: Haiyang Zhang <haiyangz@microsoft.com>
Cc: stable <stable@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>