mptcp: pm: do not remove already closed subflows

It is possible to have in the list already closed subflows, e.g. the
initial subflow has been already closed, but still in the list. No need
to try to close it again, and increments the related counters again.

Fixes: 0ee4261a ("mptcp: implement mptcp_pm_remove_subflow")
Cc: stable@vger.kernel.org
Reviewed-by: Mat Martineau <martineau@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>