LU-5163 mdd: migrated entry may not exist

LU-5163 mdd: migrated entry may not exist

During dirent migration, we shouldn't assert file exists.

Lustre-change: https://review.whamcloud.com/26620
Lustre-commit: 542b76d142c491a0a1bf8a2f4fd22af4733f59cb

Signed-off-by: Lai Siyao <lai.siyao@intel.com>
Change-Id: I11bbc5556007ec045b7a5d57a250981082ef6d70
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: John L. Hammond <john.hammond@intel.com>
Signed-off-by: Minh Diep <minh.diep@intel.com>
Reviewed-on: https://review.whamcloud.com/31281
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>