LU-11933 mdt: clear sp_cr_flags in migrate unpack

LU-11933 mdt: clear sp_cr_flags in migrate unpack

mdt_thread_info.mti_spec is not cleared after operation handling, so
mdt_migrate_unpack() should clear it in case the old values are used.

Lustre-change: https://review.whamcloud.com/36154
Lustre-commit: d4da3b55a8303d937828e74341b3ab5c4dfd52b2

Signed-off-by: Lai Siyao <lai.siyao@whamcloud.com>
Change-Id: Ib3d5d39a4a072621c8da8b6ef7869cb4d8178aac
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Li Xi <lixi@ddn.com>
Reviewed-on: https://review.whamcloud.com/36399
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>