commit 9ea2aa8b upstream.

Make sure there is enough room for the nfnetlink header in the
netlink messages that are part of the batch. There is a similar
check in netlink_rcv_skb().
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Luis Henriques <luis.henriques@canonical.com>