Avoid panic in case of pool errors and missing L2ARC

In case an ARC buffer is allocated only on L2ARC, and there are
underlying errors in a pool with the cache device in faulty state, a
panic can occur in arc_read_done()->arc_hdr_destroy()->
arc_hdr_l2arc_destroy()->arc_hdr_clear_flags() when trying to free
the ARC buffer.

Fix this by checking in arc_read_done() if the ARC buffer to be freed
is stored on L2ARC and not empty, and acquiring its hash_lock in this
case.

Signed-off-by: George Amanakis <gamanakis@gmail.com>

module/zfs/arc.c

@@ -5867,8 +5867,15 @@ arc_read_done(zio_t *zio)
 		kmem_free(acb, sizeof (arc_callback_t));
 	}
 
-	if (freeable)
+	if (freeable) {
+		if (HDR_HAS_L2HDR(hdr) && !HDR_EMPTY(hdr))
+			mutex_enter(hash_lock);
+
 		arc_hdr_destroy(hdr);
+
+		if (MUTEX_HELD(hash_lock))
+			mutex_exit(hash_lock);
+	}
 }
 
 /*