genetlink: do not validate dump requests if there is no policy

Unlike do requests, dump genetlink requests now perform strict validation
by default even if the genetlink family does not set policy and maxtype
because it does validation and parsing on its own (e.g. because it wants to
allow different message format for different commands). While the null
policy will be ignored, maxtype (which would be zero) is still checked so
that any attribute will fail validation.

The solution is to only call __nla_validate() from genl_family_rcv_msg()
if family->maxtype is set.

Fixes: ef6243ac

 ("genetlink: optionally validate strictly/dumps")
Signed-off-by: Michal Kubecek <mkubecek@suse.cz>
Reviewed-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: David S. Miller <davem@davemloft.net>