vfs: Is mounted should be testing mnt_ns for NULL or error.

commit 260a459d2e39761fbd39803497205ce1690bc7b1 upstream.

A bug was introduced with the is_mounted helper function in
commit f7a99c5b
Author: Al Viro <viro@zeniv.linux.org.uk>
Date:   Sat Jun 9 00:59:08 2012 -0400

    get rid of ->mnt_longterm

    it's enough to set ->mnt_ns of internal vfsmounts to something
    distinct from all struct mnt_namespace out there; then we can
    just use the check for ->mnt_ns != NULL in the fast path of
    mntput_no_expire()

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

The intent was to test if the real_mount(vfsmount)->mnt_ns was
NULL_OR_ERR but the code is actually testing real_mount(vfsmount)
and always returning true.

The result is d_absolute_path returning paths it should be hiding.

Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>