net: dsa: mv88e6xxx: avoid unregistering devlink regions which were never registered

Russell King reports that a system with mv88e6xxx dereferences a NULL
pointer when unbinding this driver:
https://lore.kernel.org/netdev/Z_lRkMlTJ1KQ0kVX@shell.armlinux.org.uk/

The crash seems to be in devlink_region_destroy(), which is not NULL
tolerant but is given a NULL devlink global region pointer.

At least on some chips, some devlink regions are conditionally registered
since the blamed commit, see mv88e6xxx_setup_devlink_regions_global():

		if (cond && !cond(chip))
			continue;

These are MV88E6XXX_REGION_STU and MV88E6XXX_REGION_PVT. If the chip
does not have an STU or PVT, it should crash like this.

To fix the issue, avoid unregistering those regions which are NULL, i.e.
were skipped at mv88e6xxx_setup_devlink_regions_global() time.

Fixes: 836021a ("net: dsa: mv88e6xxx: Export cross-chip PVT as devlink region")
Tested-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Link: https://patch.msgid.link/20250414212850.2953957-1-vladimir.oltean@nxp.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: vladimiroltean

drivers/net/dsa/mv88e6xxx/devlink.c

@@ -736,7 +736,8 @@ void mv88e6xxx_teardown_devlink_regions_global(struct dsa_switch *ds)
 	int i;
 
 	for (i = 0; i < ARRAY_SIZE(mv88e6xxx_regions); i++)
-		dsa_devlink_region_destroy(chip->regions[i]);
+		if (chip->regions[i])
+			dsa_devlink_region_destroy(chip->regions[i]);
 }
 
 void mv88e6xxx_teardown_devlink_regions_port(struct dsa_switch *ds, int port)