LU-15097 quota: stop pool_recalc before killing pool

LU-15097 quota: stop pool_recalc before killing pool

qmt_start_pool_recalc holds a refrence on a pool while
it is running. This thread should be stopped before
putting the last pool reference in qmt_pool_free to be
sure that pool can finally freed. Patch helps to avoid
following ASSERTION:

    qmt_pool_fini() ASSERTION(list_empty(&qmt->qmt_pool_list)) failed

Lustre-change: https://review.whamcloud.com/45256
Lustre-commit: 862f0baa7c21cb631b98d3886ef9e938f4519573

Change-Id: If72042a620d9ded693fcb669bc9148d1f96126a4
Signed-off-by: Sergey Cheremencev <sergey.cheremencev@hpe.com>
Reviewed-on: https://review.whamcloud.com/c/ex/lustre-release/+/46656
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>