sbin/fsck_msdosfs: Correct off-by-two issue when determining FAT type.

[delphij]

Summary

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.

PR: 243179

Test Plan

Create FAT16 media on a 2GiB storage and run fsck_msdosfs

[delphij]

Sync with NetBSD which added more boundary checks.

[delphij]

Reduce the patch back to original (NetBSD sync was merged).

[emaste]

OK