ipv6: fix sparse warnings in rt6_info_hash_nhsfn()

Adding by commit 51ebd318 which adds the support of ECMP for IPv6.

Spotted-by: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Signed-off-by: David S. Miller <davem@davemloft.net>