netlink: add NETLINK_CAP_ACK socket option

Since commit c05cdb1b86 ("netlink: allow large data transfers from
user-space"), the kernel may fail to allocate the necessary room for the
acknowledgment message back to userspace. This patch introduces a new
socket option that trims off the payload of the original netlink message.

The netlink message header is still included, so the user can guess from
the sequence number what is the message that has triggered the
acknowledgment.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Christophe Ricard <christophe-h.ricard@st.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/uapi/linux/netlink.h

@@ -110,6 +110,7 @@ struct nlmsgerr {
 #define NETLINK_TX_RING			7
 #define NETLINK_LISTEN_ALL_NSID		8
 #define NETLINK_LIST_MEMBERSHIPS	9
+#define NETLINK_CAP_ACK			10
 
 struct nl_pktinfo {
 	__u32	group;