generic: test two vfsmount no peers

generic: test two vfsmount no peers

This test cover linux commit 7ae8fd0, when mnt_group_id=0, it means
this mount no peers. But this bug treat two zero mnt_group_id as
peers. And it cause a crash by dereference a NULL address.

As below, the crash will happen when mount fs on "B/mnt1/mnt2":

           shared            New FS           shared
     -----------------------[A/mnt1]----------------------
    |                           |                         |
    |                bind       |    bind                 |
[C/mnt1]--[slave C]<------[shared A]------>[slave B]--[B/mnt1]
                                                          |
                                                          |
                                                    [B/mnt1/mnt2]
                                                       (New FS)

Signed-off-by: Zorro Lang <zlang@redhat.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Reviewed-by: Eryu Guan <eguan@redhat.com>
Signed-off-by: Eryu Guan <eguan@redhat.com>