mptcp: continue marking the first subflow as UNCONNECTED

After the 'Fixes' commit mentioned below, which is a partial backport,
the MPTCP worker was no longer marking the first subflow as "UNCONNECTED"
when the socket was transitioning to TCP_CLOSE state.

As a result, in v6.1, it was no longer possible to reconnect to the just
disconnected socket. Continue to do that like before, only for the first
subflow.

A few refactoring have been done around the 'msk->subflow' in later
versions, and it looks like this is not needed to do that there, but
still needed in v6.1. Without that, the 'disconnect' tests from the
mptcp_connect.sh selftest fail: they repeat the transfer 3 times by
reconnecting to the server each time.

Fixes: 7857e35e

 ("mptcp: get rid of msk->subflow")
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>