Detect exFAT filesystems and abort if found

If fsck_msdos tries to fix an exFAT filesystem, it will damage it
and it won't be mountable on systems that support exFAT.  So just
abort if the OEM name field is "EXFAT" or an invalid FAT32
Extended BIOS Parameter Block is detected.  The test to detect
an invalid BIOS Parameter Block was developed by Sony.

Change-Id: I09e8457e399239e840c825b858a635a7d9462def