msdosfs: take inusemap inconsistency as an error, not invariants violation

In other words, stop silently accepting freeing free cluster in
non-debug kernels, but return the error to the caller. Modify callers
to handle errors from usemap_free().

In collaboration with: pho
Reviewed by: markj, mckusick
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D33721