illumos gate

We ran into this: https://github.com/openzfs/zfs/pull/14358 (fix is the commit shown there).

If we receive a DRR_FREEOBJECTS as the first entry in an object range, this might end up producing a hole if the freed objects were the only existing objects in the block.

If the txg starts syncing before we've processed any following DRR_OBJECT records, this leads to a possible race where the backing arc_buf_t gets its psize set to 0 in the arc_write_ready() callback while still being referenced from a dirty record in the open txg.

To prevent this, we insert a txg_wait_synced call if the first record in the range was a DRR_FREEOBJECTS that actually resulted in one or more freed objects.

Testing done: With this patch, the system have not had panic for weeks.