generic: test bind mount operations

generic: test bind mount operations

This case will do function test for mount bind operation, it will
verify below semantics:

---------------------------------------------------------------------------
|         BIND MOUNT OPERATION                                            |
|**************************************************************************
|source(A)->| shared       |       private  |       slave    | unbindable |
| dest(B)  |               |                |                |            |
|   |      |               |                |                |            |
|   v      |               |                |                |            |
|**************************************************************************
|  shared  | shared        |     shared     | shared & slave |  invalid   |
|          |               |                |                |            |
|non-shared| shared        |      private   |      slave     |  invalid   |
***************************************************************************

This case usees fsstress to produce a small random load, to make
sure basic operations on the bind mountpoints won't cause hang or
panic etc.

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>