net/mlx5e: Properly resolve TC offloaded ipv6 vxlan tunnel source address

Currently if vxlan tunnel ipv6 src isn't supplied the driver fails to
resolve it as part of the route lookup. The resulting encap header
is left with a zeroed out ipv6 src address so the packets are sent
with this src ip.

Use an appropriate route lookup API that also resolves the source
ipv6 address if it's not supplied.

Fixes: ce99f6b9

 ('net/mlx5e: Support SRIOV TC encapsulation offloads for IPv6 tunnels')
Signed-off-by: Paul Blakey <paulb@mellanox.com>
Reviewed-by: Or Gerlitz <ogerlitz@mellanox.com>
Reviewed-by: Roi Dayan <roid@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>