vduse: avoid empty string for dev name

Syzkaller hits a kernel WARN when the first character of the dev name
provided is NULL. Solution is to add a NULL check before calling
cdev_device_add() in vduse_create_dev().

kobject: (0000000072042169): attempted to be registered with empty name!
WARNING: CPU: 0 PID: 112695 at lib/kobject.c:236
Call Trace:
 kobject_add_varg linux/src/lib/kobject.c:390 [inline]
 kobject_add+0xf6/0x150 linux/src/lib/kobject.c:442
 device_add+0x28f/0xc20 linux/src/drivers/base/core.c:2167
 cdev_device_add+0x83/0xc0 linux/src/fs/char_dev.c:546
 vduse_create_dev linux/src/drivers/vdpa/vdpa_user/vduse_dev.c:2254 [inline]
 vduse_ioctl+0x7b5/0xf30 linux/src/drivers/vdpa/vdpa_user/vduse_dev.c:2316
 vfs_ioctl linux/src/fs/ioctl.c:47 [inline]
 file_ioctl linux/src/fs/ioctl.c:510 [inline]
 do_vfs_ioctl+0x14b/0xa80 linux/src/fs/ioctl.c:697
 ksys_ioctl+0x7c/0xa0 linux/src/fs/ioctl.c:714
 __do_sys_ioctl linux/src/fs/ioctl.c:721 [inline]
 __se_sys_ioctl linux/src/fs/ioctl.c:719 [inline]
 __x64_sys_ioctl+0x42/0x50 linux/src/fs/ioctl.c:719
 do_syscall_64+0x94/0x330 linux/src/arch/x86/entry/common.c:291
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

Fixes: c8a6153b ("vduse: Introduce VDUSE - vDPA Device in Userspace")
Cc: "Xie Yongji" <xieyongji@bytedance.com>
Reported-by: Xianjun Zeng <zengxianjun@bytedance.com>
Signed-off-by: Sheng Zhao <sheng.zhao@bytedance.com>
Message-Id: <20230530033626.1266794-1-sheng.zhao@bytedance.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Reviewed-by: Xie Yongji <xieyongji@bytedance.com>
Cc: "Michael S. Tsirkin"<mst@redhat.com>, "Jason Wang"<jasowang@redhat.com>,
Reviewed-by: Xie Yongji <xieyongji@bytedance.com>

drivers/vdpa/vdpa_user/vduse_dev.c

@@ -1685,6 +1685,9 @@ static bool vduse_validate_config(struct vduse_dev_config *config)
 	if (config->vq_num > 0xffff)
 		return false;
 
+	if (!config->name[0])
+		return false;
+
 	if (!device_is_allowed(config->device_id))
 		return false;