Fix -Wuse-after-free warning in dbuf_destroy()

Move the use of the db pointer after it is freed. It's only used as
a tag so a dereference would never occur, but there's no reason we
can't invert the order to resolve the warning.

module/zfs/dbuf.c: In function 'dbuf_destroy':
module/zfs/dbuf.c:2953:17: error:
pointer 'db' may be used after 'free' [-Werror=use-after-free]

Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #13528
Closes #13575