fix IN_DELETE_SELF on overwriting rename() on ramfs et.al.

On ramfs and other simple_rename() users IN_DELETE_SELF is not generated
for victim of overwriting rename() if it's is a directory.  Works on
most of the local filesystems and really trivial to fix...
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/libfs.c

@@ -328,8 +328,10 @@ int simple_rename(struct inode *old_dir, struct dentry *old_dentry,
 
 	if (new_dentry->d_inode) {
 		simple_unlink(new_dir, new_dentry);
-		if (they_are_dirs)
+		if (they_are_dirs) {
+			drop_nlink(new_dentry->d_inode);
 			drop_nlink(old_dir);
+		}
 	} else if (they_are_dirs) {
 		drop_nlink(old_dir);
 		inc_nlink(new_dir);