Revert "ZFS_IOC_COUNT_FILLED does unnecessary txg_wait_synced()"

This reverts commit 4b3133e671b958fa2c915a4faf57812820124a7b.

Users identified this commit as a possible source of data
corruption:
https://github.com/openzfs/zfs/issues/14753

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Tony Hutter <hutter2@llnl.gov>
Issue #14753
Closes #14761