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

Move the use of the private pointer after it is freed.  It's only
used as a tag so a dereference would never occur, but there's no
harm in inverting the order to resolve the warning.

    module/zfs/dbuf.c: In function 'dbuf_issue_final_prefetch_done':
    module/zfs/dbuf.c:3204:17: error:
    pointer 'private' 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 openzfs#13528
Closes openzfs#13575

module/zfs/dbuf.c

@@ -3199,9 +3199,10 @@ dbuf_issue_final_prefetch_done(zio_t *zio, const zbookmark_phys_t *zb,
 	(void) zio, (void) zb, (void) iobp;
 	dbuf_prefetch_arg_t *dpa = private;
 
-	dbuf_prefetch_fini(dpa, B_TRUE);
 	if (abuf != NULL)
 		arc_buf_destroy(abuf, private);
+
+	dbuf_prefetch_fini(dpa, B_TRUE);
 }
 
 /*