mptcp: better msk-level shutdown.

mptcp: better msk-level shutdown.

Instead of re-implementing most of inet_shutdown, re-use
such helper, and implement the MPTCP-specific bits at the
'proto' level.

The msk-level disconnect() can now be invoked, lets provide a
suitable implementation.

As a side effect, this fixes bad state management for listener
sockets. The latter could lead to division by 0 oops since
commit 4b82918d43b2 ("mptcp: refine MPTCP-level ack scheduling").

Fixes: 93d3b913d64c ("mptcp: Use full MPTCP-level disconnect state machine")
Fixes: 4b82918d43b2 ("mptcp: refine MPTCP-level ack scheduling")
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Reviewed-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>