atomic_open(): return the right dentry in FMODE_OPENED case

->atomic_open() might have used a different alias than the one we'd
passed to it; in "not opened" case we take care of that, in "opened"
one we don't.  Currently we don't care downstream of "opened" case
which alias to return; however, that will change shortly when we
get to unifying may_open() calls.

It's not hard to get right in all cases, anyway.
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/namei.c

@@ -2949,11 +2949,15 @@ static struct dentry *atomic_open(struct nameidata *nd, struct dentry *dentry,
 	d_lookup_done(dentry);
 	if (!error) {
 		if (file->f_mode & FMODE_OPENED) {
+			int acc_mode = op->acc_mode;
+			if (unlikely(dentry != file->f_path.dentry)) {
+				dput(dentry);
+				dentry = dget(file->f_path.dentry);
+			}
 			/*
 			 * We didn't have the inode before the open, so check open
 			 * permission here.
 			 */
-			int acc_mode = op->acc_mode;
 			if (file->f_mode & FMODE_CREATED) {
 				WARN_ON(!(open_flag & O_CREAT));
 				fsnotify_create(dir, dentry);