mptcp: reset last_snd on subflow close

Send logic caches last active subflow in the msk, so it needs to be
cleared when the cached subflow is closed.

Fixes: d5f49190 ("mptcp: allow picking different xmit subflows")
Closes: https://github.com/multipath-tcp/mptcp_net-next/issues/155


Reported-by: Christoph Paasch <cpaasch@apple.com>
Acked-by: Paolo Abeni <pabeni@redhat.com>
Reviewed-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: David S. Miller <davem@davemloft.net>