atomic_open(): saner calling conventions (return dentry on success)

Currently it either returns -E... or puts (nd->path.mnt,dentry)
into *path and returns 0.  Make it return ERR_PTR(-E...) or
dentry; adjust the caller.  Fewer arguments and it's easier
to keep track of *path contents that way.
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/namei.c

@@ -3087,10 +3087,10 @@ static int may_o_create(const struct path *dir, struct dentry *dentry, umode_t m
  *
  * Returns an error code otherwise.
  */
-static int atomic_open(struct nameidata *nd, struct dentry *dentry,
-			struct path *path, struct file *file,
-			const struct open_flags *op,
-			int open_flag, umode_t mode)
+static struct dentry *atomic_open(struct nameidata *nd, struct dentry *dentry,
+				  struct file *file,
+				  const struct open_flags *op,
+				  int open_flag, umode_t mode)
 {
 	struct dentry *const DENTRY_NOT_SET = (void *) -1UL;
 	struct inode *dir =  nd->path.dentry->d_inode;
@@ -3131,17 +3131,15 @@ static int atomic_open(struct nameidata *nd, struct dentry *dentry,
 			}
 			if (file->f_mode & FMODE_CREATED)
 				fsnotify_create(dir, dentry);
-			if (unlikely(d_is_negative(dentry))) {
+			if (unlikely(d_is_negative(dentry)))
 				error = -ENOENT;
-			} else {
-				path->dentry = dentry;
-				path->mnt = nd->path.mnt;
-				return 0;
-			}
 		}
 	}
-	dput(dentry);
-	return error;
+	if (error) {
+		dput(dentry);
+		dentry = ERR_PTR(error);
+	}
+	return dentry;
 }
 
 /*
@@ -3236,11 +3234,20 @@ static int lookup_open(struct nameidata *nd, struct path *path,
 	}
 
 	if (dir_inode->i_op->atomic_open) {
-		error = atomic_open(nd, dentry, path, file, op, open_flag,
-				    mode);
-		if (unlikely(error == -ENOENT) && create_error)
-			error = create_error;
-		return error;
+		dentry = atomic_open(nd, dentry, file, op, open_flag, mode);
+		if (IS_ERR(dentry)) {
+			error = PTR_ERR(dentry);
+			if (unlikely(error == -ENOENT) && create_error)
+				error = create_error;
+			return error;
+		}
+		if (file->f_mode & FMODE_OPENED) {
+			dput(dentry);
+			return 0;
+		}
+		path->mnt = nd->path.mnt;
+		path->dentry = dentry;
+		return 0;
 	}
 
 no_open: