mptcp: avoid setting TCP_CLOSE state twice

tcp_set_state() is called from tcp_done() already.

There is then no need to first set the state to TCP_CLOSE, then call
tcp_done().

Fixes: d5824847

 ("mptcp: fix fallback for MP_JOIN subflows")
Cc: stable@vger.kernel.org
Closes: https://github.com/multipath-tcp/mptcp_net-next/issues/362
Acked-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>