LU-9404 mdt: set HSM xattr only when needed

LU-9404 mdt: set HSM xattr only when needed

In mdt_hsm_add_hal() avoid setting the HSM xattr when the HSM
attributes have not changed.

Signed-off-by: John L. Hammond <john.hammond@intel.com>
Change-Id: I12570034127b9928e49ea329bf77b674aaa6ade8
Reviewed-on: https://review.whamcloud.com/26867
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Frank Zago <fzago@cray.com>
Reviewed-by: Faccini Bruno <bruno.faccini@intel.com>
Reviewed-by: Henri Doreau <henri.doreau@cea.fr>
Reviewed-by: Quentin Bouget <quentin.bouget@cea.fr>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>