ipv6: sr: properly initialize flowi6 prior passing to ip6_route_output

In 'seg6_output', stack variable 'struct flowi6 fl6' was missing
initialization.

Fixes: 6c8702c6 ("ipv6: sr: add support for SRH encapsulation and injection with lwtunnels")
Signed-off-by: Shmulik Ladkani <shmulik.ladkani@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>