/proc/kcore: update stat.st_size after memory hotplug

After memory hotplug (or other events in future), kcore size can be
modified.

To update inode->i_size, we have to know inode/dentry but we can't get it
from inside /proc directly.  But considerinyg memory hotplug, kcore image
is updated only when it's opened.  Then, updating inode->i_size at open()
is enough.
Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Acked-by: WANG Cong <xiyou.wangcong@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/proc/kcore.c

@@ -546,6 +546,11 @@ static int open_kcore(struct inode *inode, struct file *filp)
 		return -EPERM;
 	if (kcore_need_update)
 		kcore_update_ram();
+	if (i_size_read(inode) != proc_root_kcore->size) {
+		mutex_lock(&inode->i_mutex);
+		i_size_write(inode, proc_root_kcore->size);
+		mutex_unlock(&inode->i_mutex);
+	}
 	return 0;
 }