LU-14324 tests: Fix fsx fallocate detection

LU-14324 tests: Fix fsx fallocate detection

Avoid kernel playing fake tricks on us by optimizing away small
increases in file size by fallocate, so to ensure the truth make
large fallocate call instead.
Additionally always truncate to zero after all call, as even a
failed call can result in increased file size.

Test-Parameters: trivial testlist=sanity env=ONLY=150
Test-Parameters: testlist=sanityn env=ONLY=16,COUNT=10000
Test-Parameters: testlist=sanity-dom env=ONLY=fsx,sanityn,SANITYN_ONLY=16
Test-Parameters: fstype=zfs testlist=sanity env=ONLY=150
Test-Parameters: fstype=zfs testlist=sanityn env=ONLY=16,COUNT=10000
Test-Parameters: fstype=zfs testlist=sanity-dom env=ONLY=fsx,sanityn,SANITYN_ONLY=16
Change-Id: I8cddb7d0d30bdf89bf13d18ae36a5f950adc392d
Fixes: 03247bda765c ("LU-14286 osd-ldiskfs: fallocate() should zero new blocks")
Signed-off-by: Oleg Drokin <green@whamcloud.com>
Reviewed-on: https://review.whamcloud.com/41202
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Arshad Hussain <arshad.hussain@aeoncomputing.com>
Tested-by: Maloo <maloo@whamcloud.com>