userns: nfnetlink_log: Report socket uids in the log sockets user namespace

At logging instance creation capture the peer netlink socket's user
namespace. Use the captured peer user namespace when reporting socket
uids to the peer.

The peer socket's user namespace is guaranateed to be valid until the user
closes the netlink socket.  nfnetlink_log removes instances during the final
close of a socket.  __build_packet_message does not get called after an
instance is destroyed.   Therefore it is safe to let the peer netlink socket
take care of the user namespace reference counting for us.

Acked-by: David S. Miller <davem@davemloft.net>
Acked-by: Serge Hallyn <serge.hallyn@canonical.com>
Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>