Get rid of mnt_mountpoint abuses in ext4

path to mnt/mnt->mnt_root is no worse than that to
mnt->mnt_parent/mnt->mnt_mountpoint *and* needs no
pinning the sucker down (mnt is not going away and
mnt->mnt_root won't change)
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/ext4/file.c

@@ -116,11 +116,9 @@ static int ext4_file_open(struct inode * inode, struct file * filp)
 		 * devices or filesystem images.
 		 */
 		memset(buf, 0, sizeof(buf));
-		path.mnt = mnt->mnt_parent;
-		path.dentry = mnt->mnt_mountpoint;
-		path_get(&path);
+		path.mnt = mnt;
+		path.dentry = mnt->mnt_root;
 		cp = d_path(&path, buf, sizeof(buf));
-		path_put(&path);
 		if (!IS_ERR(cp)) {
 			memcpy(sbi->s_es->s_last_mounted, cp,
 			       sizeof(sbi->s_es->s_last_mounted));