fs: fat: first dentry of long name in FAT iterator

A long name is split over multiple directory entries. When deleting a file
with a long name we need the first directory entry to be able to delete the
whole chain.

Add the necessary fields to the FAT iterator:

* cluster of first directory entry
* address of first directory entry
* remaining entries in cluster

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>