ipv6: use lwtunnel_output6() only if flag redirect is set

This function make sense only when LWTUNNEL_STATE_OUTPUT_REDIRECT is set.
The check is already done in IPv4.

CC: Thomas Graf <tgraf@suug.ch>
CC: Roopa Prabhu <roopa@cumulusnetworks.com>
Fixes: 74a0f2fe ("ipv6: rt6_info output redirect to tunnel output")
Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Acked-by: Thomas Graf <tgraf@suug.ch>
Acked-by: Roopa Prabhu <roopa@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>