LU-15839 tests: correct the ZFS grace time for sanity-quota 4a

LU-15839 tests: correct the ZFS grace time for sanity-quota 4a

For  sanity-quota 4a, the grace time is increased from 12s to 20s but
not actually set on filesystem.

Fixes: 3e4c3fdc ("LU-6836 test: re-add test 4a to sanity-quota for ZFS")
Test-Parameters: fstype=zfs testlist=sanity-quota env=ONLY=4a,ONLY_REPEAT=100
Signed-off-by: Etienne AUJAMES <eaujames@ddn.com>
Change-Id: I2324e818a42a19bc9928f127b1622f1e5274db1f
Reviewed-on: https://review.whamcloud.com/47289
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Sergey Cheremencev <sergey.cheremencev@hpe.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>