Use MUTEX_FSTRANS on l2arc_buflist_mtx

Use MUTEX_FSTRANS on l2arc_buflist_mtx to prevent the following deadlock
scenario:
1. arc_release() -> hash_lock -> l2arc_buflist_mtx
2. l2arc_write_buffers() -> l2arc_buflist_mtx -> (direct reclaim) ->
   arc_buf_remove_ref() -> hash_lock

Signed-off-by: Chunwei Chen <tuxoko@gmail.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Signed-off-by: Tim Chase <tim@chase2k.com>
Issue openzfs#3160

module/zfs/arc.c

@@ -5530,7 +5530,7 @@ l2arc_init(void)
 	mutex_init(&l2arc_feed_thr_lock, NULL, MUTEX_DEFAULT, NULL);
 	cv_init(&l2arc_feed_thr_cv, NULL, CV_DEFAULT, NULL);
 	mutex_init(&l2arc_dev_mtx, NULL, MUTEX_DEFAULT, NULL);
-	mutex_init(&l2arc_buflist_mtx, NULL, MUTEX_DEFAULT, NULL);
+	mutex_init(&l2arc_buflist_mtx, NULL, MUTEX_FSTRANS, NULL);
 	mutex_init(&l2arc_free_on_write_mtx, NULL, MUTEX_DEFAULT, NULL);
 
 	l2arc_dev_list = &L2ARC_dev_list;