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

tests/zfs-tests/tests/functional/block_cloning/block_cloning_copyfilerange_fallback_same_txg.ksh

@@ -52,6 +52,8 @@ log_must set_tunable64 TXG_TIMEOUT 5000
 
 log_must zpool create -o feature@block_cloning=enabled $TESTPOOL $DISKS
 
+log_must sync_pool $TESTPOOL true
+
 log_must dd if=/dev/urandom of=/$TESTPOOL/file bs=128K count=4
 log_must clonefile -f /$TESTPOOL/file /$TESTPOOL/clone 0 0 524288