mptcp: implement fastclose xmit path

mptcp: implement fastclose xmit path

Allow the MPTCP xmit path to add MP_FASTCLOSE suboption
on RST egress packets.

Additionally reorder related options writing to reduce
the number of conditionals required in the fast path.

Co-developed-by: Geliang Tang <[email protected]>
Signed-off-by: Geliang Tang <[email protected]>
Co-developed-by: Matthieu Baerts <[email protected]>
Signed-off-by: Matthieu Baerts <[email protected]>
Signed-off-by: Paolo Abeni <[email protected]>
Signed-off-by: Mat Martineau <[email protected]>
Signed-off-by: David S. Miller <[email protected]>