LU-7988 hsm: make mdt_hsm_cdt_start a static function

LU-7988 hsm: make mdt_hsm_cdt_start a static function

It's only used in the file it's declared in.

Test-Parameters: trivial
Signed-off-by: Frank Zago <fzago@cray.com>
Change-Id: Ic0b3af0c18cfb09f2f94efbac814429f0c43cb4b
Reviewed-on: http://review.whamcloud.com/22704
Reviewed-by: James Simmons <uja.ornl@yahoo.com>
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: John L. Hammond <john.hammond@intel.com>
Reviewed-by: Vinayak <vinayakswami.hariharmath@seagate.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>