cxgbi: fix uninitialized flowi6

ip6_route_output looks into different fields in the passed flowi6 structure,
yet cxgbi passes garbage in nearly all those fields. Zero the structure out
first.

Fixes: fc8d0590

 ("libcxgbi: Add ipv6 api to driver")
Signed-off-by: Jiri Benc <jbenc@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>