Expand Fletcher-16 NMAX

The limit when calculating the value of NMAX is 2^16-1 only when sum1
and sum2 are represented as 16-bit. We're representing them as 32-bit.

This will be helpful for SIMD implementations of Fletcher-16, because
the old limit of 22 bytes would have meant that we could only process
16 bytes at a time before having to compute modulo.

fletcher32.h

@@ -20,7 +20,7 @@
 #define AARU_CHECKSUMS_NATIVE_FLETCHER32_H
 
 #define FLETCHER32_MODULE 0xFFFF
-/* NMAX is the largest n such that 255n(n+1)/2 + (n+1)(ADLER_MODULE-1) <= 2^32-1 */
+/* NMAX is the largest n such that 255n(n+1)/2 + (n+1)(FLETCHER32_MODULE-1) <= 2^32-1 */
 #define NMAX 5552
 
 typedef struct