Merge branch 'mptcp-lowat-sockopt'

Matthieu Baerts says:

====================
mptcp: add TCP_NOTSENT_LOWAT sockopt support

Patch 3 does the magic of adding TCP_NOTSENT_LOWAT support, all the
other ones are minor cleanup seen along when working on the new
feature.

Note that this feature relies on the existing accounting for snd_nxt.
Such accounting is not 110% accurate as it tracks the most recent
sequence number queued to any subflow, and not the actual sequence
number sent on the wire. Paolo experimented a lot, trying to implement
the latter, and in the end it proved to be both "too complex" and "not
necessary".

The complexity raises from the need for additional lock and a lot of
refactoring to introduce such protections without adding significant
overhead. Additionally, snd_nxt is currently used and exposed with the
current semantic by the internal packet scheduling. Introducing a
different tracking will still require us to keep the old one.

More interestingly, a more accurate tracking could be not strictly
necessary: as the MPTCP socket enqueues data to the subflows only up
to the available send window, any enqueue data is sent on the wire
instantly, without any blocking operation short or a drop in the tx
path at the nft or TC layer.
====================

Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>