devlink: hold region lock when flushing snapshots

Netdevsim triggers a splat on reload, when it destroys regions
with snapshots pending:

  WARNING: CPU: 1 PID: 787 at net/core/devlink.c:6291 devlink_region_snapshot_del+0x12e/0x140
  CPU: 1 PID: 787 Comm: devlink Not tainted 6.1.0-07460-g7ae9888d #580
  RIP: 0010:devlink_region_snapshot_del+0x12e/0x140
  Call Trace:
   <TASK>
   devl_region_destroy+0x70/0x140
   nsim_dev_reload_down+0x2f/0x60 [netdevsim]
   devlink_reload+0x1f7/0x360
   devlink_nl_cmd_reload+0x6ce/0x860
   genl_family_rcv_msg_doit.isra.0+0x145/0x1c0

This is the locking assert in devlink_region_snapshot_del(),
we're supposed to be holding the region->snapshot_lock here.

Fixes: 2dec18ad ("net: devlink: remove region snapshots list dependency on devlink->lock")
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/devlink.c

@@ -11925,8 +11925,10 @@ void devl_region_destroy(struct devlink_region *region)
 	devl_assert_locked(devlink);
 
 	/* Free all snapshots of region */
+	mutex_lock(&region->snapshot_lock);
 	list_for_each_entry_safe(snapshot, ts, &region->snapshot_list, list)
 		devlink_region_snapshot_del(region, snapshot);
+	mutex_unlock(&region->snapshot_lock);
 
 	list_del(&region->list);
 	mutex_destroy(&region->snapshot_lock);