Fix copy and paste error setting rt_ttl_propagate.

Fixes: 5b441ac8 ("mpls: allow TTL propagation to IP packets to be configured")
Signed-off-by: NDavid Ahern <dsa@cumulusnetworks.com>
Acked-by: NRobert Shearman <rshearma@brocade.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>