LU-17588 tests: test_block_soft: set max_diry_mb=1

LU-17588 tests: test_block_soft: set max_diry_mb=1

Set max_dirty_mb to 1 during the test in test_block_soft.
This affects 3a, 3b and 3c. Sometimes all written data
could be stored in a cache because tests 3a 3b and 3c
write quite small amount of data - from 4 to 6 MB.
If there was no BRW at OST side, tests would fail with:

  Grace timeout was not set or quota not exceeded

In debug logs from sanity-quota_1g failure have been found
that BRW size at OST sometimes could be 4MB despite of
osc*.*.max_diry_mb set to 1MB. This could be the reason of
failure because the file in 1g has overstriping and one of
OSTs might not reply EDQUOT to the moment of last write.
Set oflag=sync to avoid that.

Test-Parameters: trivial testlist=sanity-quota env=ONLY=3,ONLY_REPEAT=100
Fixes: de352465eb ("LU-17046 tests: fix write success in 1g")
Signed-off-by: Sergey Cheremencev <scherementsev@ddn.com>
Change-Id: I0f430051b3f9f37a0c628ea33e108f9bd42838ed
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/54208
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Hongchao Zhang <hongchao@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>