random: do not sign extend bytes for rotation when mixing

By using `char` instead of `unsigned char`, certain platforms will sign
extend the byte when `w = rol32(*bytes++, input_rotate)` is called,
meaning that bit 7 is overrepresented when mixing. This isn't a real
problem (unless the mixer itself is already broken) since it's still
invertible, but it's not quite correct either. Fix this by using an
explicit unsigned type.
Signed-off-by: NJason A. Donenfeld <Jason@zx2c4.com>

drivers/char/random.c

@@ -546,7 +546,7 @@ static void _mix_pool_bytes(struct entropy_store *r, const void *in,
 	unsigned long i, tap1, tap2, tap3, tap4, tap5;
 	int input_rotate;
 	int wordmask = r->poolinfo->poolwords - 1;
-	const char *bytes = in;
+	const unsigned char *bytes = in;
 	__u32 w;
 
 	tap1 = r->poolinfo->tap1;