LU-15740 tests: scale fs_log_size by OSTCOUNT

LU-15740 tests: scale fs_log_size by OSTCOUNT

The fs_log_size "free space skew" was being scaled by MDSCOUNT,
but in fact this parameter is only ever used to compare the OST
free space usage, so the OSTCOUNT should be used when scaling it.

It is likely that the skew is actually caused by blocks allocated
by OST object directories and not llogs (no llogs used on OSTs for
many years), but it isn't worthwhile to rename the function.

Lustre-change: https://review.whamcloud.com/50419
Lustre-commit: fabec6f2cb39950a2f208567dac716e21880fa9f

Test-Parameters: trivial testlist=replay-single env=ONLY="20b 89"
Test-Parameters: testlist=runtests clientdistro=ubuntu2204
Test-Parameters: testlist=replay-ost-single env=ONLY="6 7"
Test-Parameters: testlist=sanity-sec env=ONLY="16-22
Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Change-Id: I97f05b10fa7ec367534b5bdce09feae5e93ebbe5
Reviewed-by: Arshad Hussain <arshad.hussain@aeoncomputing.com>
Reviewed-by: Alex Deiter <alex.deiter@gmail.com>
Reviewed-on: https://review.whamcloud.com/c/ex/lustre-release/+/52742
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>