tests/block_cloning: sync before write in fallback test

We're still seeing this test fail intermittently (that is, the clone
happens), which must mean the write and the clone can still be happening
on different txgs.

It might be that there's still activity after the pool is created. So
here we force a sync before starting the write.

Sponsored-By: Klara Inc.
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Rob Norris <rob.norris@klarasystems.com>
Closes #15359