mptcp: pm: only set request_bkup flag when sending MP_PRIO

commit 4258b948 upstream.

The 'backup' flag from mptcp_subflow_context structure is supposed to be
set only when the other peer flagged a subflow as backup, not the
opposite.

Fixes: 06706542 ("mptcp: add the outgoing MP_PRIO support")
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, because the commit f5360e9b ("mptcp:
  introduce and use mptcp_pm_send_ack()") is not in this version. This
  code is in mptcp_pm_nl_mp_prio_send_ack() instead of in a dedicated
  helper. The same modification can be applied there. ]
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>