Fix cloning into already dirty dbufs.

Undirty the dbuf and destroy its buffer when cloning into it.

Coverity ID: CID-1535375
Reported-by: Richard Yao
Reported-by: Benjamin Coddington
Reviewed-by: Richard Yao <richard.yao@alumni.stonybrook.edu>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Signed-off-by: Pawel Jakub Dawidek <pawel@dawidek.net>
Closes #14655