mptcp: move subflow close loop after sk close check

In case mptcp socket is already dead the entire mptcp socket
will be freed. We can avoid the close check in this case.

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>