LU-15694 quota: keep grace time while setting default limits

LU-15694 quota: keep grace time while setting default limits

The quota grace time should only be changed by "lfs setquota -t",
and it should be kept while setting default quota limits.

This patch also fixes an issue of not saving the grace time while
writing glboal quota record.

Lustre-change: https://review.whamcloud.com/46935
Lustre-commit: d4978678b49102226a79a6c8e5d10075d416977d

Signed-off-by: Hongchao Zhag <hongchao@whamcloud.com>
Change-Id: I89ca49d09dc41deffe4bc77e53721b5bb4f4be37
Reviewed-on: https://review.whamcloud.com/48416
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>