mtdchar: fix integer overflow in read/write ioctls

In the Linux kernel, the following vulnerability has been resolved: mtdchar: fix integer overflow in read/write ioctls The "req.start" and "req.len" variables are u64 values that come from the user at the start of the function. We mask away the high 32 bits of "req.len" so that's capped at U32_MAX but the "req.start" variable can go up to U64_MAX which means that the addition can still integer overflow. Use check_add_overflow() to fix this bug.