LU-12895 mdt: check if object exists first

LU-12895 mdt: check if object exists first

Make sure object exists before trying to get its attr.

Lustre-change: https://review.whamcloud.com/36629
Lustre-commit: ca68e3d677a371497586167a2318268db1d94cab

Test-Parameters: clientselinux mdtcount=4 envdefinitions=ONLY=185a testlist=sanity,sanity,sanity,sanity
Test-Parameters: clientselinux mdtcount=4 testlist=sanity,recovery-small,sanity-sec
Signed-off-by: Sebastien Buisson <sbuisson@ddn.com>
Change-Id: Idb2cd5d6e3fdf7998040b933be54a001a0e5391b
Reviewed-on: https://review.whamcloud.com/37032
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>