LU-9312 hsm: fix error handling around mdt_hsm_get_md_hsm()

LU-9312 hsm: fix error handling around mdt_hsm_get_md_hsm()

Correct several spurious NULL return checks from
mdt_hsm_get_md_hsm().

Signed-off-by: John L. Hammond <john.hammond@intel.com>
Change-Id: Icfe74e87183bc5356d4c7627088b402805dcc164
Reviewed-on: https://review.whamcloud.com/26741
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Faccini Bruno <bruno.faccini@intel.com>
Reviewed-by: Henri Doreau <henri.doreau@cea.fr>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>