Merge branch 'mptcp-fixes-for-ulp-a-deadlock-and-netlink-docs'

Mat Martineau says:

====================
mptcp: Fixes for ULP, a deadlock, and netlink docs

Two of the MPTCP fixes in this set are related to the TCP_ULP socket
option with MPTCP sockets operating in "fallback" mode (the connection
has reverted to regular TCP). The other issues are an observed deadlock
and missing parameter documentation in the MPTCP netlink API.

Patch 1 marks TCP_ULP as unsupported earlier in MPTCP setsockopt code,
so the fallback code path in the MPTCP layer does not pass the TCP_ULP
option down to the subflow TCP socket.

Patch 2 makes sure a TCP fallback socket returned to userspace by
accept()ing on a MPTCP listening socket does not allow use of the
"mptcp" TCP_ULP type. That ULP is intended only for use by in-kernel
MPTCP subflows.

Patch 3 fixes the possible deadlock when sending data and there are
socket option changes to sync to the subflows.

Patch 4 makes sure all MPTCP netlink event parameters are documented
in the MPTCP uapi header.
====================

Link: https://lore.kernel.org/r/20211214231604.211016-1-mathew.j.martineau@linux.intel.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>