generic: test mount move semantics

generic: test mount move semantics

This's a functional test case for mount --move operation, it verifies
below semantics:

  ---------------------------------------------------------------------------
  |         MOVE MOUNT OPERATION                                            |
  |**************************************************************************
  |source(A)->| shared       |       private  |       slave    | unbindable |
  | dest(B)  |               |                |                |            |
  |   |      |               |                |                |            |
  |   v      |               |                |                |            |
  |**************************************************************************
  |  shared  | shared        |     shared     | shared & slave |  invalid   |
  |          |               |                |                |            |
  |non-shared| shared        |      private   |      slave     | unbindable |
  ***************************************************************************
  NOTE: moving a mount residing under a shared mount is invalid.

This case uses fsstress to produce a little random load, to make
sure basic operations won't break the the moved mountpoints.

Signed-off-by: Zorro Lang <zlang@redhat.com>
Reviewed-by: Eryu Guan <guaneryu@gmail.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>