annotate_ecksum() asserts that nui64s, calculated as nui64s = size / sizeof
(uint64_t), is not greater than UINT16_MAX.
This restriction is needed because histograms of incorrectly set and cleared
bits have 16 bit counters and if the buffer consists of too many 64-bit words,
then a counter can potentially overflow producing an incorrect result.
When the largest buffer size was 128KB the greatest value of nui64s was 16K,
well within the limit.
But now we have support for large buffers and for buffer sizes of 512KB and
above the restriction is violated.