The char buffer buf, receives data directly from user space,
so its content might be negative and its elements are left
shifted to form an unsigned integer.

Since left shifting a negative value is undefined behavior, thus
change the char to u8 to elimintate this UB.
Signed-off-by: Changming Liu <charley.ashbringer@gmail.com>
Link: https://lore.kernel.org/r/20200711043018.928-1-charley.ashbringer@gmail.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>