LU-6449 hsm: handle completions with non-existent objects

LU-6449 hsm: handle completions with non-existent objects

In hsm_cdt_request_completed() release the layout lock for a completed
restore regardless of whether the object existed. Rewrite
hsm_swap_layouts() to accept the already found object rather than its
FID. Add a comment to mdt_object_unlock() that the object passed may
be NULL.

Signed-off-by: John L. Hammond <john.hammond@intel.com>
Signed-off-by: Bruno Faccini <bruno.faccini@intel.com>
Change-Id: I571df2dab25822ba5a4e40c60018871051140f93
Reviewed-on: https://review.whamcloud.com/22212
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Mike Pershin <mike.pershin@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>