clone_private_mount() doesn't need to touch namespace_sem

not for CL_PRIVATE clone_mnt()
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/namespace.c

@@ -1795,9 +1795,7 @@ struct vfsmount *clone_private_mount(struct path *path)
 	if (IS_MNT_UNBINDABLE(old_mnt))
 		return ERR_PTR(-EINVAL);
 
-	down_read(&namespace_sem);
 	new_mnt = clone_mnt(old_mnt, path->dentry, CL_PRIVATE);
-	up_read(&namespace_sem);
 	if (IS_ERR(new_mnt))
 		return ERR_CAST(new_mnt);