LU-15503 quota: fix list entry usage

LU-15503 quota: fix list entry usage

Fetch next list entry.

Lustre-change: https://review.whamcloud.com/46380
Lustre-commit: TBD (from 48903591fe31735391564ee36ca6723f2aa5ac18)

Fixes: d527e81246 (LU-15283 quota: deadlock between reint & lquota_wb)

Test-Parameters: trivial mdtcount=4 mdscount=2 testlist=sanity-quota

Signed-off-by: Yang Sheng <ys@whamcloud.com>
Change-Id: I86befdfaa96151a6fd61902ffbf43ee8e5cae8cb
Reviewed-on: https://review.whamcloud.com/46396
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>