Comment boot block checks and perform additional sanity checks:
The following checks are now being enforced:
- bpbBytesPerSec: only accept 512, 1024, 2048 and 4096.
- bpbSecPerClust: only accept 1, 2, 4, 8, 16, 32, 64 and 128.
- bpbResSectors: require non-zero.
- bpbFATs: require non-zero.
- bpbSectors: require zero for FAT32.
- bpbFATsmall: require zero for FAT32.
- bpbHugeSectors: require non-zero for FAT32.
Bail out if the BPB contained values that do not meet these requirements.
We also require FATsecs * FATsecs to not overflow 32-bit unsigned
Check for backup boot block was removed because the checker does not take
corrective action, and msdosfs driver ignores it too.