LU-9663 tests: replace “ls -s” with “stat -c '%s'” to get file size

This patch improves acl/2561{,_zfs}.test to use “stat -c '%s'” to
get file size instead of using “ls -s”, which prints the allocated
size of the file in blocks.

Test-Parameters: trivial \
envdefinitions=ONLY=103 \
mdtfilesystemtype=zfs ostfilesystemtype=zfs \
testlist=sanity

Change-Id: I8bfea5197102baeece244e20731c90172c5cc2f1
Signed-off-by: Jian Yu <jian.yu@intel.com>
Reviewed-on: https://review.whamcloud.com/27961
Tested-by: Jenkins
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Fan Yong <fan.yong@intel.com>