dpaa2-switch: prevent ZERO_SIZE_PTR dereference when num_ifs is zero

The driver allocates arrays for ports, FDBs, and filter blocks using
kcalloc() with ethsw->sw_attr.num_ifs as the element count. When the
device reports zero interfaces (either due to hardware configuration
or firmware issues), kcalloc(0, ...) returns ZERO_SIZE_PTR (0x10)
instead of NULL.

Later in dpaa2_switch_probe(), the NAPI initialization unconditionally
accesses ethsw->ports[0]->netdev, which attempts to dereference
ZERO_SIZE_PTR (address 0x10), resulting in a kernel panic.

Add a check to ensure num_ifs is greater than zero after retrieving
device attributes. This prevents the zero-sized allocations and
subsequent invalid pointer dereference.

Reported-by: Yuhao Jiang <danisjiang@gmail.com>
Reported-by: Junrui Luo <moonafterrain@outlook.com>
Fixes: 0b1b71370458 ("staging: dpaa2-switch: handle Rx path on control interface")
Signed-off-by: Junrui Luo <moonafterrain@outlook.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://patch.msgid.link/SYBPR01MB7881BEABA8DA896947962470AF91A@SYBPR01MB7881.ausprd01.prod.outlook.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/ethernet/freescale/dpaa2/dpaa2-switch.c

diff --git a/drivers/net/ethernet/freescale/dpaa2/dpaa2-switch.c b/drivers/net/ethernet/freescale/dpaa2/dpaa2-switch.c
index b1e1ad9..0ff234f 100644
--- a/drivers/net/ethernet/freescale/dpaa2/dpaa2-switch.c
+++ b/drivers/net/ethernet/freescale/dpaa2/dpaa2-switch.c
@@ -3024,6 +3024,12 @@ static int dpaa2_switch_init(struct fsl_mc_device *sw_dev)
 		goto err_close;
 	}
 
+	if (!ethsw->sw_attr.num_ifs) {
+		dev_err(dev, "DPSW device has no interfaces\n");
+		err = -ENODEV;
+		goto err_close;
+	}
+
 	err = dpsw_get_api_version(ethsw->mc_io, 0,
 				   &ethsw->major,
 				   &ethsw->minor);