reftable/writer: add check for limits

We check for max update index in reftable_writer_add_ref but not in reftable_writer_add_log, so we need to add the same.

Tried adding this in one of my patch series: 67bd2a7b, but a lot of tests failed:

Job #8400590899 failed for 102b5a15

So it is little more involved.