fs: umount on symlink leaks mnt count

Currently umount on symlink blocks following umount:

/vz is separate mount

# ls /vz/ -al | grep test
drwxr-xr-x.  2 root root       4096 Jul 19 01:14 testdir
lrwxrwxrwx.  1 root root         11 Jul 19 01:16 testlink -> /vz/testdir
# umount -l /vz/testlink
umount: /vz/testlink: not mounted (expected)

# lsof /vz
# umount /vz
umount: /vz: device is busy. (unexpected)

In this case mountpoint_last() gets an extra refcount on path->mnt

Signed-off-by: Vasily Averin <vvs@openvz.org>
Acked-by: Ian Kent <raven@themaw.net>
Acked-by: Jeff Layton <jlayton@primarydata.com>
Cc: stable@vger.kernel.org
Signed-off-by: Christoph Hellwig <hch@lst.de>
user_path_parent(int dfd, const char __user *path, struct nameidata *nd,
		 unsigned int flags)
{
	struct filename *s = getname(path);
	int error;

	/* only LOOKUP_REVAL is allowed in extra flags */
	flags &= LOOKUP_REVAL;

	if (IS_ERR(s))
		return s;

	error = filename_lookup(dfd, s, flags | LOOKUP_PARENT, nd);
	if (error) {
		putname(s);
		return ERR_PTR(error);
	}

	return s;
}

user_path_parent(int dfd, const char __user *path, struct nameidata *nd,
		 unsigned int flags)
{
	struct filename *s = getname(path);
	int error;

	/* only LOOKUP_REVAL is allowed in extra flags */
	flags &= LOOKUP_REVAL;

	if (IS_ERR(s))
		return s;

	error = filename_lookup(dfd, s, flags | LOOKUP_PARENT, nd);
	if (error) {
		putname(s);
		return ERR_PTR(error);
	}

	return s;
}
