HAMMER fsstress panic in hammer_setup_child_callback
While running fsstress on HAMMER on a build of DragonFly from today (after the
most recent dsched fixes, with fq policy), I got a panic in
hammer_setup_child_callback, called from hammer_rec_rb_tree_RB_SCAN, ultimately
from hammer_flusher_slave_thread. Fsync_mode = 2.