Add dns_rdataset_cleanup() that conditionally disassociate rdataset

Add dns_rdataset_cleanup() that conditionally disassociate rdataset

We had a common pattern in the code that looks like this:

	if (dns_rdataset_isassociated(rdataset)) {
		dns_rdataset_disassociate(rdataset);
	}

add a helper macro that checks for rdataset != NULL and the above
called dns_rdataset_cleanup(rdataset).