tools/testing/vma: Implement vm_refcnt reset

Add the reset of the ref count in vma_lock_init().  This is needed if
the vma memory is not zeroed on allocation.

Signed-off-by: Liam R. Howlett <Liam.Howlett@oracle.com>
Reviewed-by: Suren Baghdasaryan <surenb@google.com>
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>