Merge branch 'mptcp-multiple-subflows-path-management'

Mat Martineau says:

====================
Multipath TCP part 3: Multiple subflows and path management

v2 -> v3: Remove 'inline' in .c files, fix uapi bit macros, and rebase.

v1 -> v2: Rebase on current net-next, fix for netlink limit setting,
and update .gitignore for selftest.

This patch set allows more than one TCP subflow to be established and
used for a multipath TCP connection. Subflows are added to an existing
connection using the MP_JOIN option during the 3-way handshake. With
multiple TCP subflows available, sent data is now stored in the MPTCP
socket so it may be retransmitted on any TCP subflow if there is no
DATA_ACK before a timeout. If an MPTCP-level timeout occurs, data is
retransmitted using an available subflow. Storing this sent data
requires the addition of memory accounting at the MPTCP level, which was
previously delegated to the single subflow. Incoming DATA_ACKs now free
data from the MPTCP-level retransmit buffer.

IP addresses available for new subflow connections can now be advertised
and received with the ADD_ADDR option, and the corresponding REMOVE_ADDR
option likewise advertises that an address is no longer available.

The MPTCP path manager netlink interface has commands to set in-kernel
limits for the number of concurrent subflows and control the
advertisement of IP addresses between peers.

To track and debug MPTCP connections there are new MPTCP MIB counters,
and subflow context can be requested using inet_diag. The MPTCP
self-tests now validate multiple-subflow operation and the netlink path
manager interface.
====================

Signed-off-by: David S. Miller <davem@davemloft.net>