udp: multicast packets need to check namespace

[ Upstream commit 920a46115ca3fa88990276d98520abab85495b2d ]

Current UDP multicast delivery is not namespace aware.

Signed-off-by: Eric Dumazet <dada1@cosmosbay.com>
Acked-by: Pavel Emelyanov <xemul@openvz.org>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>