net: Break struct flowi out into AF specific instances.

Now we have struct flowi4, flowi6, and flowidn for each address
family.  And struct flowi is just a union of them all.

It might have been troublesome to convert flow_cache_uli_match() but
as it turns out this function is completely unused and therefore can
be simply removed.

Signed-off-by: David S. Miller <davem@davemloft.net>