afs: Fix double-unmap of directory block

In the Linux kernel, the following vulnerability has been resolved: afs: Fix double-unmap of directory block Fix afs_edit_dir_remove() to use a cleanup function to unmap the block pointed to by afs_dir_iter::block if it's left pointing to something rather than manually kunmapping the blocks. Manually kunmapping without clearing iter.blocks can result in a double-kunmap if afs_dir_find_block() is called twice in a row (which would be the case if the block being modified is not first in the hash chain).