genetlink: correct uAPI defines

Commit 50a896cf ("genetlink: properly support per-op policy dumping")
seems to have copy'n'pasted things a little incorrectly.

The #define CTRL_ATTR_MCAST_GRP_MAX should have stayed right
after the previous enum. The new CTRL_ATTR_POLICY_* needs
its own define for MAX and that max should not contain the
superfluous _DUMP in the name.

We probably can't do anything about the CTRL_ATTR_POLICY_DUMP_MAX
any more, there's likely code which uses it. For consistency
(*cough* codegen *cough*) let's add the correctly name define
nonetheless.

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Reviewed-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: David S. Miller <davem@davemloft.net>