procfs: return ENOENT on opening a being-removed proc entry

Change the return value to ENOENT.  This return value is then returned
when opening the proc entry that have been removed.  For example,
open("/proc/bus/pci/XX/YY") when the corresponding device is being
hot-removed.

Signed-off-by: Daisuke Ogino <ogino.daisuke@jp.fujitsu.com>
Cc: Jesse Barnes <jbarnes@virtuousgeek.org>
Acked-by: Alexey Dobriyan <adobriyan@gmail.com>
Cc: Christoph Hellwig <hch@lst.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/proc/inode.c

@@ -319,7 +319,7 @@ static int proc_reg_open(struct inode *inode, struct file *file)
 	if (!pde->proc_fops) {
 		spin_unlock(&pde->pde_unload_lock);
 		kfree(pdeo);
-		return -EINVAL;
+		return -ENOENT;
 	}
 	pde->pde_users++;
 	open = pde->proc_fops->open;