The "fat" file-system mount type doesn't exist in Linux. Trying to mount a
FAT16 partition with "fat" file-system type returns -ENODEV.
Fixes: <https://issues.guix.gnu.org/48419>.
* gnu/installer/parted.scm (user-fs-type->mount-type): Use the "vfat"
file-system for FAT16 partitions.