LU-17527 tests: fix sanity/255a decimal comparison

LU-17527 tests: fix sanity/255a decimal comparison

Don't use fractional percentages in the sanity test_255a performance
comparison, since bash (( ... )) cannot compare numbers with decimal
points properly.  Instead, just compute the percentage speedup with
whole numbers, since the test discards anything less than 20% speedup
and a fraction of a percent will not make much difference here.

Test-Parameters: trivial
Fixes: bdd470ff97 ("LU-9069 tests: improve output of sanity test_255a")
Fixes: 395f3e1a55 ("LU-15316 tests: use integers in sanity test_255a")
Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Change-Id: Id3b37e07168ee2590e52d01f66336027254ced55
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/56836
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Arshad Hussain <arshad.hussain@aeoncomputing.com>
Reviewed-by: Jian Yu <yujian@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>