[FASTFAT] Fix FastFAT not returning short name for FAT volumes in FileBothDirectoryIn...

[FASTFAT] Fix FastFAT not returning short name for FAT volumes in FileBothDirectoryInformation case
This is likely due to a copy paste error where long name was copied twice and short never.
Fun fact: this was not affecting FATX volumes
Fun fact2: this was defeating a buffer overflow check and thus was allowing buffer overflow!

CORE-14088