fat: reset VFAT short alias checksum on first match

The VFAT short alias checksum read from a long file name is only overwritten
when another long file name appears in a directory list. Until then it renders
short file names invisible that have the same checksum. Reset the checksum on
first match.

Signed-off-by: Sergei Shtylyov <sshtylyov@ru.mvista.com>
Signed-off-by: Martin Mueller <martin.mueller5@de.bosch.com>