generic: test mount shared subtrees state transition

generic: test mount shared subtrees state transition

This case will do function test for mount --make-* operations, it
will verify below state transition:

------------------------------------------------------------------------
|             |make-shared |  make-slave  | make-private |make-unbindab|
--------------|------------|--------------|--------------|-------------|
|shared       |shared      |*slave/private|   private    | unbindable  |
|             |            |              |              |             |
|-------------|------------|--------------|--------------|-------------|
|slave        |shared      |    **slave   |    private   | unbindable  |
|             |and slave   |              |              |             |
|-------------|------------|--------------|--------------|-------------|
|shared       |shared      |    slave     |    private   | unbindable  |
|and slave    |and slave   |              |              |             |
|-------------|------------|--------------|--------------|-------------|
|private      |shared      |  **private   |    private   | unbindable  |
|-------------|------------|--------------|--------------|-------------|
|unbindable   |shared      |**unbindable  |    private   | unbindable  |
------------------------------------------------------------------------

This case uses fsstress to produce a small random load, to make sure
basic operations on the 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>