blkid: Handle short/empty vfat labels; update tests

vfat labels have a fixed space allocated for them. In the case of a
shorter label, the remaining bytes are padded with spaces. A vfat
filesystem with no label (i.e. that will show up in Windows as "Local
Disk (X:)" or "Removable Disk (X:)") is stored as "NO NAME    ".

Both of these changes match behavior from util-linux.
2 files changed