net: dsa: bcm_sf2: Fix node reference count

of_find_node_by_name() will do an of_node_put() on the "from" argument.
With CONFIG_OF_DYNAMIC enabled which checks for device_node reference
counts, we would be getting a warning like this:

[    6.347230] refcount_t: increment on 0; use-after-free.
[    6.352498] WARNING: CPU: 3 PID: 77 at lib/refcount.c:156
refcount_inc_checked+0x38/0x44
[    6.360601] Modules linked in:
[    6.363661] CPU: 3 PID: 77 Comm: kworker/3:1 Tainted: G        W
5.4.46-gb78b3e9956e6 #13
[    6.372546] Hardware name: BCM97278SV (DT)
[    6.376649] Workqueue: events deferred_probe_work_func
[    6.381796] pstate: 60000005 (nZCv daif -PAN -UAO)
[    6.386595] pc : refcount_inc_checked+0x38/0x44
[    6.391133] lr : refcount_inc_checked+0x38/0x44
...
[    6.478791] Call trace:
[    6.481243]  refcount_inc_checked+0x38/0x44
[    6.485433]  kobject_get+0x3c/0x4c
[    6.488840]  of_node_get+0x24/0x34
[    6.492247]  of_irq_find_parent+0x3c/0xe0
[    6.496263]  of_irq_parse_one+0xe4/0x1d0
[    6.500191]  irq_of_parse_and_map+0x44/0x84
[    6.504381]  bcm_sf2_sw_probe+0x22c/0x844
[    6.508397]  platform_drv_probe+0x58/0xa8
[    6.512413]  really_probe+0x238/0x3fc
[    6.516081]  driver_probe_device+0x11c/0x12c
[    6.520358]  __device_attach_driver+0xa8/0x100
[    6.524808]  bus_for_each_drv+0xb4/0xd0
[    6.528650]  __device_attach+0xd0/0x164
[    6.532493]  device_initial_probe+0x24/0x30
[    6.536682]  bus_probe_device+0x38/0x98
[    6.540524]  deferred_probe_work_func+0xa8/0xd4
[    6.545061]  process_one_work+0x178/0x288
[    6.549078]  process_scheduled_works+0x44/0x48
[    6.553529]  worker_thread+0x218/0x270
[    6.557285]  kthread+0xdc/0xe4
[    6.560344]  ret_from_fork+0x10/0x18
[    6.563925] ---[ end trace 68f65caf69bb152a ]---

Fix this by adding a of_node_get() to increment the reference count
prior to the call.

Fixes: afa3b592 ("net: dsa: bcm_sf2: Ensure correct sub-node is parsed")
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/dsa/bcm_sf2.c

@@ -1147,6 +1147,8 @@ static int bcm_sf2_sw_probe(struct platform_device *pdev)
 	set_bit(0, priv->cfp.used);
 	set_bit(0, priv->cfp.unique);
 
+	/* Balance of_node_put() done by of_find_node_by_name() */
+	of_node_get(dn);
 	ports = of_find_node_by_name(dn, "ports");
 	if (ports) {
 		bcm_sf2_identify_ports(priv, ports);