deal with races between remove_proc_entry() and proc_reg_release()

* serialize the call of ->release() on per-pdeo mutex
* don't remove pdeo from per-pde list until we are through with it

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/proc/internal.h

@@ -153,6 +153,8 @@ int proc_readdir_de(struct proc_dir_entry *de, struct file *filp, void *dirent,
 struct pde_opener {
 	struct file *file;
 	struct list_head lh;
+	int count;	/* number of threads in close_pdeo() */
+	struct mutex mutex;
 };
 
 ssize_t __proc_file_read(struct file *, char __user *, size_t, loff_t *);