mptcp: pm: only set request_bkup flag when sending MP_PRIO

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>