LU-14494 mdt: check object exists in mdt_close_handle_layouts()

LU-14494 mdt: check object exists in mdt_close_handle_layouts()

In mdt_close_handle_layouts() the client supplied FID may not identify
an existing object. So check for this before calling lu_object_attr().

Signed-off-by: John L. Hammond <jhammond@whamcloud.com>
Change-Id: Ib1710ca4bf7587e0496b3a37a2afb65f81250455
Reviewed-on: https://review.whamcloud.com/41905
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Alex Zhuravlev <bzzz@whamcloud.com>
Reviewed-by: Mike Pershin <mpershin@whamcloud.com>