fat: root directory cluster only makes sense for FAT32

The root directory cluster field only exists in a FAT32 boot sector, so the
'root_cluster' variable in do_fat_read() contains garbage in case of FAT12/16.
Make it contain 0 instead as this is what is passed to get_vfatname() in that
case anyway.

Signed-off-by: Sergei Shtylyov <sshtylyov@mvista.com>