LU-15129 tests: sanity-quota_75_dom fix

LU-15129 tests: sanity-quota_75_dom fix

oflag=sync causes dd to write page by page instead
of sending several pages in RPC. Furthermore, when
granted space becomes closer to soft_limit(i.e. over
9MB if soft_limit is 10MB), OST can not preacquire
space anymore. Also OST could acquire only requested
amount of space - see qmt_alloc_expand. Thus OST has
to send quota acquire request at MDT for each BRW request
from the client. Sometimes 20 seconds is not enough to write
10MB. Change oflag=sync with conv=fsync to reduce the number
of RPCs between the client and OST and between QSDs and QMT.
One fsync at close should help to avoid timeout failure.

Lustre-change: https://review.whamcloud.com/50164
Lustre-commit: 7d05a687ee5d4f4b95585244a7f60394475fe0ba

Test-Parameters: trivial testlist=sanity-quota
Test-Parameters: testlist=sanity-quota env=ONLY=75,ONLY_REPEAT=50
Signed-off-by: Sergey Cheremencev <scherementsev@ddn.com>
Change-Id: Iad363fdc8a0984861055c295ea9cc3f23110fd9f
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Arshad Hussain <arshad.hussain@aeoncomputing.com>
Reviewed-on: https://review.whamcloud.com/c/ex/lustre-release/+/50336
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>