netlink: Use netlink header as base to calculate bad attribute offset

netlink: Use netlink header as base to calculate bad attribute offset

[ Upstream commit 88316239d096822f8ce64588eae8805bca2d6b10 ]

Userspace might send a batch that is composed of several netlink
messages. The netlink_ack() function must use the pointer to the netlink
header as base to calculate the bad attribute offset.

Fixes: 3ec49c377cb6 ("netlink: extended ACK reporting")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>