Merge branch 'mptcp-support-changes-to-initial-subflow-priority'

Mat Martineau says:

====================
mptcp: Support changes to initial subflow priority

This series updates the in-kernel MPTCP path manager to allow changes to
subflow priority for the first subflow created for each MPTCP connection
(the one created with the MP_CAPABLE handshake).

Patches 1 and 2 do some refactoring to simplify the new functionality.

Patch 3 introduces the new feature to change the initial subflow
priority and send the MP_PRIO header on that subflow.

Patch 4 cleans up code related to tracking endpoint ids on the initial
subflow.

Patch 5 adds a selftest to confirm that subflow priorities are updated
as expected.
====================

Link: https://lore.kernel.org/r/20220711191633.80826-1-mathew.j.martineau@linux.intel.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>