radix-tree: fix deleting a multi-order entry through an alias

If we deleted an entry through an index which looked up a sibling
pointer, we'd end up zeroing out the wrong slots in the node.  Use
get_slot_offset() to find the right slot.
Signed-off-by: Matthew Wilcox <willy@linux.intel.com>
Reviewed-by: Ross Zwisler <ross.zwisler@linux.intel.com>
Cc: Konstantin Khlebnikov <koct9i@gmail.com>
Cc: Kirill Shutemov <kirill.shutemov@linux.intel.com>
Cc: Jan Kara <jack@suse.com>
Cc: Neil Brown <neilb@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

lib/radix-tree.c

@@ -1558,7 +1558,7 @@ void *radix_tree_delete_item(struct radix_tree_root *root,
 		return entry;
 	}
 
-	offset = index & RADIX_TREE_MAP_MASK;
+	offset = get_slot_offset(node, slot);
 
 	/*
 	 * Clear all tags associated with the item to be deleted.