Merge branch 'mptcp-re-enable-sndbuf-autotune'

Paolo Abeni says:

====================
mptcp: re-enable sndbuf autotune

The sendbuffer autotuning was unintentionally disabled as a
side effect of the recent workqueue removal refactor. These
patches re-enable id, with some extra care: with autotuning
enable/large send buffer we need a more accurate packet
scheduler to be able to use efficiently the available
subflow bandwidth, especially when the subflows have
different capacities.

The first patch cleans-up subflow socket handling, making
the actual re-enable (patch 2) simpler.

Patches 3 and 4 improve the packet scheduler, to better cope
with non trivial scenarios and large send buffer.

Finally patch 5 adds and uses some infrastructure to avoid
the workqueue usage for the packet scheduler operations introduced
by the previous patches.
====================

Link: https://lore.kernel.org/r/cover.1611153172.git.pabeni@redhat.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>