mptcp: use mptcp_for_each_subflow in mptcp_close

This patch used the macro helper mptcp_for_each_subflow() instead of
list_for_each_entry() in mptcp_close.

Signed-off-by: Geliang Tang <geliangtang@gmail.com>
Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>