genetlink: use parsed attrs in dumppolicy

Attributes are already parsed based on the policy specified
in the family and ready-to-use in info->attrs. No need to
call genlmsg_parse() again.

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