LU-12581 osc: prevent use after free

LU-12581 osc: prevent use after free

Clear aa_oa after it's been freed to prevent use after free.

Lustre-change: https://review.whamcloud.com/35601
Lustre-commit: 61c9f8797771c951ecd240981d7d97d5adc685e0

Signed-off-by: Bobi Jam <bobijam@whamcloud.com>
Change-Id: Idf122aa53fe5b13c07337745e5a26763e8712be2
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Patrick Farrell <pfarrell@whamcloud.com>
Signed-off-by: Minh Diep <mdiep@whamcloud.com>
Reviewed-on: https://review.whamcloud.com/36210
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>