Merge branch 'mptcp-mptcp-support-for-tcp_fastopen_connect'

Mat Martineau says:

====================
mptcp: MPTCP support for TCP_FASTOPEN_CONNECT

RFC 8684 appendix B describes how to use TCP Fast Open with MPTCP. This
series allows TFO use with MPTCP using the TCP_FASTOPEN_CONNECT socket
option. The scope here is limited to the initiator of the connection -
support for MSG_FASTOPEN and the listener side of the connection will be
in a separate series. The preexisting TCP fastopen code does most of the
work, so these changes mostly involve plumbing MPTCP through to those
TCP functions.

Patch 1 changes the MPTCP socket option code to pass the
TCP_FASTOPEN_CONNECT option through to the initial unconnected subflow.

Patch 2 exports the existing tcp_sendmsg_fastopen() function from tcp.c

Patch 3 adds the call to tcp_sendmsg_fastopen() from the MPTCP send
function.

Patch 4 modifies mptcp_poll() to handle the deferred TFO connection.
====================

Link: https://lore.kernel.org/r/20220926232739.76317-1-mathew.j.martineau@linux.intel.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>