netfilter: nfnetlink_log: account for netlink header size

netfilter: nfnetlink_log: account for netlink header size

This is a followup to an old bug fix: NLMSG_DONE needs to account
for the netlink header size, not just the attribute size.

This can result in a WARN splat + drop of the netlink message,
but other than this there are no ill effects.

Fixes: 9dfa1dfe4d5e ("netfilter: nf_log: account for size of NLMSG_DONE attribute")
Reported-by: Yiming Qian <yimingqian591@gmail.com>
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>