vme: Fix error not catched in fake_init()

In fake_init(), __root_device_register() is possible to fail but it's
ignored, which can cause unregistering vme_root fail when exit.

 general protection fault,
 probably for non-canonical address 0xdffffc000000008c
 KASAN: null-ptr-deref in range [0x0000000000000460-0x0000000000000467]
 RIP: 0010:root_device_unregister+0x26/0x60
 Call Trace:
  <TASK>
  __x64_sys_delete_module+0x34f/0x540
  do_syscall_64+0x38/0x90
  entry_SYSCALL_64_after_hwframe+0x63/0xcd

Return error when __root_device_register() fails.

Fixes: 658bcda ("vme: Adding Fake VME driver")
Signed-off-by: Chen Zhongjin <chenzhongjin@huawei.com>
Link: https://lore.kernel.org/r/20221205084805.147436-1-chenzhongjin@huawei.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: Chen Zhongjin

drivers/staging/vme_user/vme_fake.c

@@ -1070,6 +1070,8 @@ static int __init fake_init(void)
 
 	/* We need a fake parent device */
 	vme_root = __root_device_register("vme", THIS_MODULE);
+	if (IS_ERR(vme_root))
+		return PTR_ERR(vme_root);
 
 	/* If we want to support more than one bridge at some point, we need to
 	 * dynamically allocate this so we get one per device.