recv_recovery_from_checkpoint_start(): Clear the recv_no_ibuf_operations
flag at the same time when we enabled writes to the log.

The failure to clear the flag might have caused some missed
change buffer merges, at least to the secondary index of SYS_TABLES
that were accessed by trx_resurrect_table_locks() while the last
recovery batch was in progress.

Thanks to Thirunarayanan Balathandayuthapani for suggesting this fix.