LU-4728 mdt: fix NULL deference of mdt_fid_lock

When enabling hsm_control, mti_exp field of struct mdt_thread_info
could be NULL.  ldlm_cli_enqueue_local will crash the kernel when
dereference it.

Signed-off-by: Li Xi <lixi@ddn.com>
Change-Id: I4bdb8222aec378e396e7f544834f6fcf9fdaf777
Reviewed-on: http://review.whamcloud.com/9543
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Faccini Bruno <bruno.faccini@intel.com>
Reviewed-by: John L. Hammond <john.hammond@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>