msg.dst_sk needs to be set up with a valid socket because some callbacks
later derive the netns from it.

Fixes: 263ea09084d172d ("Revert "genl: Add genlmsg_new_unicast() for unicast message allocation")
Reported-by: NJon Maloy <maloy@donjonn.com>
Bisected-by: NJon Maloy <maloy@donjonn.com>
Signed-off-by: NFlorian Westphal <fw@strlen.de>
Acked-by Jon Maloy <jon.maloy@ericsson.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>