copy_from_user返回值：成功复制返回0，不成功复制则返回没有成功复制剩下的字节数
./app
[ 298.706261] test_chrdev_open
[ 298.707817] test_chrdev_write
[ 298.710645] copy_from_user success..
[ 298.714185] test_chrdev_read
[ 298.717041] copy_to_user success..
[ 298.720420] test_chrdev_release
open /dev/test success..
read_buf:helloworld2222.