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 openzfs#13528
Closes openzfs#13575

module/zfs/dbuf.c

@@ -2941,9 +2941,6 @@ dbuf_destroy(dmu_buf_impl_t *db)
 	ASSERT3U(db->db_caching_status, ==, DB_NO_CACHE);
 	ASSERT(!multilist_link_active(&db->db_cache_link));
 
-	kmem_cache_free(dbuf_kmem_cache, db);
-	arc_space_return(sizeof (dmu_buf_impl_t), ARC_SPACE_DBUF);
-
 	/*
 	 * If this dbuf is referenced from an indirect dbuf,
 	 * decrement the ref count on the indirect dbuf.
@@ -2952,6 +2949,9 @@ dbuf_destroy(dmu_buf_impl_t *db)
 		mutex_enter(&parent->db_mtx);
 		dbuf_rele_and_unlock(parent, db, B_TRUE);
 	}
+
+	kmem_cache_free(dbuf_kmem_cache, db);
+	arc_space_return(sizeof (dmu_buf_impl_t), ARC_SPACE_DBUF);
 }
 
 /*