ipv6 Use get_hash_from_flowi6 for rt6 hash

In rt6_info_hash_nhsfn replace the custom hashing over flowi6 that is
using xor with a call to common function get_hash_from_flowi6.

Signed-off-by: Tom Herbert <tom@herbertland.com>
Signed-off-by: David S. Miller <davem@davemloft.net>