BRT: Fix slop space calculation with block cloning
Similar to deduplication, the size of data duplicated by block cloning
should not be included in the slop space calculation.
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Signed-off-by: Yuxin Wang <yuxinwang9999@gmail.com>
Closes #15874