Tighten FAT checks and fix off-by-one error in corner case.

sbin/fsck_msdosfs/fat.c:

• readfat:
  • Only truncate out-of-range cluster pointers (1, or greater than NumClusters but smaller than CLUST_RSRVD), as the current cluster may contain some data. We can't fix reserved cluster pointers at this pass, because we do no know the potential cluster preceding it.
  • Accept valid cluster for head bitmap. This is a no-op, and mainly to improve code readability, because the 1 is already handled in the previous else if block.
• truncate_at: absorbed into checkchain.
• checkchain: save the previous node we have traversed in case that we have a chain that ends with a special (>= CLUST_RSRVD) cluster, or is free. In these cases, we need to truncate at the cluster preceding the current cluster, as the current cluster contains a marker instead of a next pointer and can not be changed to CLUST_EOF (the else case can happen if the user answered "no" at some point in readfat()).
• clearchain: correct the iterator for next cluster so that we don't stop after clearing the first cluster.
• checklost: If checkchain() thinks the chain have no cluster, it doesn't make sense to reconnect it, so don't bother asking.

Reviewed by: kevlo
MFC after: 24 days
X-MFC-With: r356313
Differential Revision: https://reviews.freebsd.org/D23065