tcp: parse mptcp options contained in reset packets

tcp: parse mptcp options contained in reset packets

Because TCP-level resets only affect the subflow, there is a MPTCP
option to indicate that the MPTCP-level connection should be closed
immediately without a mptcp-level fin exchange.

This is the 'MPTCP fast close option'.  It can be carried on ack
segments or TCP resets.  In the latter case, its needed to parse mptcp
options also for reset packets so that MPTCP can act accordingly.

Next patch will add receive side fastclose support in MPTCP.

Acked-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>