libblkid: Fix false detection of DFSee created filesystems.

libblkid: Fix false detection of DFSee created filesystems.

OS/2 and DFSee creates a pseudo FAT-12/16 header in the first 512
bytes of a filesystem which looks enough like a FAT-12/16 to fool
blkid.  Part of this is because we don't require ms_magic or vs_magic
to be the strings "FAT12 ", "FAT16 ", or "FAT32 ", since some FAT
filesystem formatters don't set ms_magic or vs_magic.  To address
this, we explicitly test for "JFS     " and "HPFS    " in ms_magic,
and if they are found, we assume the filesystem is definitely not
a FAT filesystem.

Addresses-Launchpad-Bug: #255255

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>