msdosfs: Never allocate clusters outside the volume

msdosfs: Never allocate clusters outside the volume

- Assert that usemap_alloc() and usemap_free() cluster number argument
  is valid.
- In chainlength(), return 0 if cluster start is after the max cluster.
- In chainlength(), cut the calculated cluster chain length at the max
  cluster.

Adapted from FreeBSD commit 097a1d5fbb7990980f8f806c6878537c964adf32

ok miod@