LU-2342 tests: account for log size in replay-single/20b

LU-2342 tests: account for log size in replay-single/20b

Account for larger ondisk log size in ZFS (256) vs. ldiskfs (50).

Test-Parameters: mdsfilesystemtype=zfs ostfilesystemtype=zfs   mdtfilesystemtype=zfs testlist=replay-single
Signed-off-by: Nathaniel Clark <nathaniel.l.clark@intel.com>
Change-Id: I4150d58556240b128c02ba667c4c390c79b8a463
Reviewed-on: http://review.whamcloud.com/5666
Tested-by: Hudson
Reviewed-by: Mike Pershin <mike.pershin@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>