Merge branch 'mptcp-more-miscellaneous-mptcp-fixes'

Mat Martineau says:

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mptcp: More miscellaneous MPTCP fixes

Here's another batch of fixup and enhancement patches that we have
collected in the MPTCP tree.

Patch 1 removes an unnecessary flag and related code.

Patch 2 fixes a bug encountered when closing fallback sockets.

Patches 3 and 4 choose a better transmit subflow, with a self test.

Patch 5 adjusts tracking of unaccepted subflows

Patches 6-8 improve handling of long ADD_ADDR options, with a test.

Patch 9 more reliably tracks the MPTCP-level window shared with peers.

Patch 10 sends MPTCP-level acknowledgements more aggressively, so the
peer can send more data without extra delay.
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Link: https://lore.kernel.org/r/20201119194603.103158-1-mathew.j.martineau@linux.intel.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>