LU-834 echo_client: fix page_is_vmlocked

LU-834 echo_client: fix page_is_vmlocked

->cpo_is_vmlocked() should return -EBUSY if page is locked,
or -ENODATA otherwise. Fix it for echo_client implementation.

Change-Id: I863591463fefd0c2afba32d04b62b983103a4e3d
Signed-off-by: Jinshan Xiong <jinshan.xiong@whamcloud.com>
Reviewed-on: http://review.whamcloud.com/1686
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Johann Lombardi <johann@whamcloud.com>
Reviewed-by: Niu Yawei <niu@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>