connector: allow multiple messages to be sent in one packet

This increases the amount of bundling to reduce the number of packets
sent.  For the one wire use there can be multiple struct
w1_netlink_cmd in a struct w1_netlink_msg and multiple of those in
struct cn_msg, and with this change multiple of those in a struct
nlmsghdr, and at each level the len identifies there being multiple of
the next.

Signed-off-by: David Fries <David@Fries.net>
Acked-by: Evgeniy Polyakov <zbr@ioremap.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>