LU-14370 quota: use dt_sync() to flush pending writes

LU-14370 quota: use dt_sync() to flush pending writes

and find remaining quota.

Fixes: 18cd3e1e28 ("LU-12702 quota: wait pending write before acquiring remotely")
Signed-off-by: Alex Zhuravlev <bzzz@whamcloud.com>
Change-Id: I8f1ec334a1d1eefc385d8c6ef451de8a3f12365f
Reviewed-on: https://review.whamcloud.com/41322
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Wang Shilong <wshilong@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>