BRT: Clear bv_entcount_dirty on destroy

This fixes assertion in brt_sync_table() on debug builds when last
cloned block on the vdev is freed and bv_meta_dirty is cleared,
while bv_entcount_dirty is not. Should not matter in production.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Alexander Motin <mav@FreeBSD.org>
Sponsored by: iXsystems, Inc.
Closes #16791