netfilter: nfnetlink: validate nfnetlink header from batch

commit 9ea2aa8b7dba9e99544c4187cc298face254569f 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: Greg Kroah-Hartman <gregkh@linuxfoundation.org>