staging: lustre: Prevent duplicate CT registrations

Associate copytool registration to a given MDC import so that
multiple mounts of the same filesystem do not lead to having the
copytool registered multiple time.

Signed-off-by: Henri Doreau <henri.doreau@cea.fr>
Intel-bug-id: https://jira.hpdd.intel.com/browse/LU-3882
Reviewed-on: http://review.whamcloud.com/7612


Reviewed-by: John L. Hammond <john.hammond@intel.com>
Reviewed-by: Jinshan Xiong <jinshan.xiong@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>
Signed-off-by: James Simmons <jsimmons@infradead.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>