gre: Don't accidentally set RTO_ONLINK in gre_fill_metadata_dst()

commit f7716b31 upstream.

Mask the ECN bits before initialising ->flowi4_tos. The tunnel key may
have the last ECN bit set, which will interfere with the route lookup
process as ip_route_output_key_hash() interpretes this bit specially
(to restrict the route scope).

Found by code inspection, compile tested only.

Fixes: 962924fa

 ("ip_gre: Refactor collect metatdata mode tunnel xmit to ip_md_tunnel_xmit")
Signed-off-by: Guillaume Nault <gnault@redhat.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>