mptcp: pm: skip connecting to already established sf

commit bc19ff57 upstream.

The lookup_subflow_by_daddr() helper checks if there is already a
subflow connected to this address. But there could be a subflow that is
closing, but taking time due to some reasons: latency, losses, data to
process, etc.

If an ADD_ADDR is received while the endpoint is being closed, it is
better to try connecting to it, instead of rejecting it: the peer which
has sent the ADD_ADDR will not be notified that the ADD_ADDR has been
rejected for this reason, and the expected subflow will not be created
at the end.

This helper should then only look for subflows that are established, or
going to be, but not the ones being closed.

Fixes: d84ad049 ("mptcp: skip connecting the connected address")
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>
[ Conflicts in pm_netlink.c, due to commit 4638de5a ("mptcp: handle
  local addrs announced by userspace PMs"), not in this version, and
  changing the context. The same fix can still be applied. ]
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>