proc: fix return value of proc_reg_open() in "too late" case

If ->open() wasn't called, returning 0 is misleading and, theoretically,
oopsable:
1) remove_proc_entry clears ->proc_fops, drops lock,
2) ->open "succeeds",
3) ->release oopses, because it assumes ->open was called (single_release()).
Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>

fs/proc/inode.c

@@ -342,7 +342,7 @@ static int proc_reg_open(struct inode *inode, struct file *file)
 	if (!pde->proc_fops) {
 		spin_unlock(&pde->pde_unload_lock);
 		kfree(pdeo);
-		return rv;
+		return -EINVAL;
 	}
 	pde->pde_users++;
 	open = pde->proc_fops->open;