misc: pci_endpoint_test: Fix array underflow in pci_endpoint_test_ioctl()

Commit eefb837 ("misc: pci_endpoint_test: Add doorbell test case")
added NO_BAR (-1) to the pci_barno enum which, in practical terms,
changes the enum from an unsigned int to a signed int.  If the user
passes a negative number in pci_endpoint_test_ioctl() then it results in
an array underflow in pci_endpoint_test_bar().

Fixes: eefb837 ("misc: pci_endpoint_test: Add doorbell test case")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Signed-off-by: Manivannan Sadhasivam <mani@kernel.org>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Link: https://patch.msgid.link/aIzzZ4vc6ZrmM9rI@suswa

Author: Dan Carpenter

drivers/misc/pci_endpoint_test.c

@@ -937,7 +937,7 @@ static long pci_endpoint_test_ioctl(struct file *file, unsigned int cmd,
 	switch (cmd) {
 	case PCITEST_BAR:
 		bar = arg;
-		if (bar > BAR_5)
+		if (bar <= NO_BAR || bar > BAR_5)
 			goto ret;
 		if (is_am654_pci_dev(pdev) && bar == BAR_0)
 			goto ret;