Fix reading FAT32 root dirs that span >1 cluster

Fix reading FAT32 root dirs that span >1 cluster

Unlike FAT12/16, FAT32 does not guarantee that root directories will
occupy consecutive clusters. To get each cluster after the first you
must access the FAT as with any other directory. This patch implements
this and bumps up the largest valid cluster number to support large
e.g. 64MB partitions.