LU-11135 mdt: LASSERT(lu_object_exists(o)) fails

LU-11135 mdt: LASSERT(lu_object_exists(o)) fails

mdt_object_find() can return a vaild nonexisting object.
It's return value needs to be checked additionaly if exists.

Change-Id: Ib1f5bd5289a69e29437db520706591929bf55830
Cray-bug-id: LUS-6192
Signed-off-by: Andriy Skulysh <c17819@cray.com>
Reviewed-on: https://review.whamcloud.com/32803
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Alexandr Boyko <c17825@cray.com>
Reviewed-by: Andrew Perepechko <c17827@cray.com>
Reviewed-by: Alex Zhuravlev <bzzz@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>