LU-13599 mdt: fix mti_big_lmm buffer usage

LU-13599 mdt: fix mti_big_lmm buffer usage

The mti_big_lmm buffer can be used just as temporary buffer
in some cases. It should drop mti_big_lmm_used flag after
that to avoid assertion in mdt_big_attr_get().

This fix is extracted from bigger patch of LU-11025 in
master branch.

Lustre-change: https://review.whamcloud.com/37284
Lustre-commit: a336d7c7c1cd62a5a5213835aa85b8eaa87b076a

Signed-off-by: Mikhail Pershin <mpershin@whamcloud.com>
Change-Id: I3718d6c413ef1d5f8242e548868602ef6476006e
Reviewed-on: https://review.whamcloud.com/39521
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Stephane Thiell <sthiell@stanford.edu>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Lai Siyao <lai.siyao@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>