net: dsa: fix panic when removing unoffloaded port from bridge

If a bridged port is not offloaded to the hardware - either because the
underlying driver does not implement the port_bridge_{join,leave} ops,
or because the operation failed - then its dp->bridge pointer will be
NULL when dsa_port_bridge_leave() is called. Avoid dereferncing NULL.

This fixes the following splat when removing a port from a bridge:

 Unable to handle kernel access to user memory outside uaccess routines at virtual address 0000000000000000
 Internal error: Oops: 96000004 [#1] PREEMPT_RT SMP
 CPU: 3 PID: 1119 Comm: brctl Tainted: G           O      5.17.0-rc4-rt4 #1
 Call trace:
  dsa_port_bridge_leave+0x8c/0x1e4
  dsa_slave_changeupper+0x40/0x170
  dsa_slave_netdevice_event+0x494/0x4d4
  notifier_call_chain+0x80/0xe0
  raw_notifier_call_chain+0x1c/0x24
  call_netdevice_notifiers_info+0x5c/0xac
  __netdev_upper_dev_unlink+0xa4/0x200
  netdev_upper_dev_unlink+0x38/0x60
  del_nbp+0x1b0/0x300
  br_del_if+0x38/0x114
  add_del_if+0x60/0xa0
  br_ioctl_stub+0x128/0x2dc
  br_ioctl_call+0x68/0xb0
  dev_ifsioc+0x390/0x554
  dev_ioctl+0x128/0x400
  sock_do_ioctl+0xb4/0xf4
  sock_ioctl+0x12c/0x4e0
  __arm64_sys_ioctl+0xa8/0xf0
  invoke_syscall+0x4c/0x110
  el0_svc_common.constprop.0+0x48/0xf0
  do_el0_svc+0x28/0x84
  el0_svc+0x1c/0x50
  el0t_64_sync_handler+0xa8/0xb0
  el0t_64_sync+0x17c/0x180
 Code: f9402f00 f0002261 f9401302 913cc021 (a9401404)
 ---[ end trace 0000000000000000 ]---

Fixes: d3eed0e5 ("net: dsa: keep the bridge_dev and bridge_num as part of the same structure")
Signed-off-by: Alvin Šipraga <alsi@bang-olufsen.dk>
Reviewed-by: Vladimir Oltean <olteanv@gmail.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Link: https://lore.kernel.org/r/20220221203539.310690-1-alvin@pqrs.dkSigned-off-by: Jakub Kicinski <kuba@kernel.org>

net/dsa/port.c

@@ -395,10 +395,17 @@ void dsa_port_bridge_leave(struct dsa_port *dp, struct net_device *br)
 		.tree_index = dp->ds->dst->index,
 		.sw_index = dp->ds->index,
 		.port = dp->index,
-		.bridge = *dp->bridge,
 	};
 	int err;
 
+	/* If the port could not be offloaded to begin with, then
+	 * there is nothing to do.
+	 */
+	if (!dp->bridge)
+		return;
+
+	info.bridge = *dp->bridge;
+
 	/* Here the port is already unbridged. Reflect the current configuration
 	 * so that drivers can program their chips accordingly.
 	 */