msdosfs: sanity check sector count from BPB

We use sector count to size the FAT inuse bitset. If sector count is
corrupted, kernel might be tricked into doing unbound allocation.
Ensure that the sector count does not exceed the actual volume size.

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