Introduce a new function to abstract the knowledge of whether hashed
routing and filter tables are supported for a given IPA instance.

IPA v4.2 is the only one that doesn't support hashed tables (now
and for the foreseeable future), but the name of the helper function
is better for explaining what's going on.
Signed-off-by: Alex Elder <elder@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>