fs: fat: dentry iterator for fill_dir_slot()

fs: fat: dentry iterator for fill_dir_slot()

For reusing deleted directory entries we have to adjust the function called
to step to the next directory entry.

This patch alone is not enough to actually reuse deleted directory entries
as the fill_dir_slot() is still called with first never used directory
entry.

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