msdosfs: fix cluster limit when mounting FAT-16 file systems

The maximum cluster number was calculated based on the number of data
cluters that fit in the givem partition size and the size of the FAT
area. This limit did not take into account that the highest 10 cluster
numbers are reserved and must not be used for files.

PR: 280347
MFC after: 3 days
Reported by: pho@FreeBSD.org