From FreeBSD: https://reviews.freebsd.org/D23082:
Correct off-by-two issue when determining FAT type.
In the code we used NumClusters as the upper (non-inclusive) boundary
of valid cluster number, so the actual value was 2 (CLUST_FIRST) more
than the real number of clusters. This causes a FAT16 media with
65524 clusters be treated as FAT32 and might affect FAT12 media with
4084 clusters as well.
To fix this, we increment NumClusters by CLUST_FIRST after the type
determination.
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