Merge branch 'mptcp-add-mp_prio-support-and-rework-local-address-ids'

Mat Martineau says:

====================
MPTCP: Add MP_PRIO support and rework local address IDs

Patches 1 and 2 rework the assignment of local address IDs to allow them
to be assigned by a userspace path manager, and add corresponding self
tests.

Patches 2-8 add the ability to change subflow priority after a subflow
has been established. Each subflow in a MPTCP connection has a priority
level: "regular" or "backup". Data should only be sent on backup
subflows if no regular subflows are available. The priority level can be
set when the subflow connection is established (as was already
implemented), or during the life of the connection by sending MP_PRIO in
the TCP options (as added here). Self tests are included.
====================

Link: https://lore.kernel.org/r/20210109004802.341602-1-mathew.j.martineau@linux.intel.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>