MFC	r268632:
fsck_msdosfs: Assorted fixes from other BSDs.

When truncating cluster chains fix the length of the cluster head.
http://marc.info/?t=140304310700005&r=1&w=2

Avoid infinite loops in cluster chain linked lists.
http://marc.info/?l=openbsd-tech&m=140275150804337&w=2

Avoid off-by-one on FAT12 filesystems.
http://marc.info/?l=openbsd-tech&m=140234174104724&w=2

Obtained from:	NetBSD (from OpenBSD)