Ensure that cluster allocations never allocate clusters outside the
volume limits. In particular:

• 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.
• For true paranoia, after the pm_inusemap is calculated in fillinusemap(), reset all bits in the array for clusters after the max cluster, as in-use.

Reported and tested by: pho
Sponsored by: The FreeBSD Foundation
MFC after: 2 weeks