LU-5761 tests: fix test_89 to use fs_log_size()

LU-5761 tests: fix test_89 to use fs_log_size()

The test_89 checks should use fs_log_size() to determine how much
space might be leaked "normally" (due to log files, etc), and how
much data should be written to ensure that we do not misinterpret
this as the leak of block.

Also, fix up fs_log_size() to use the correct grant_block_size
units, which are in bytes, but fs_log_size() returns size in KB.
Allow a margin of 2 large blocks to be allocated for ZFS.

Test-Parameters: trivial ostfilesystemtype=zfs mdtfilesystemtype=zfs testlist=replay-single,replay-single,replay-single
Signed-off-by: Andreas Dilger <andreas.dilger@intel.com>
Change-Id: Id55175fc7f25fea52345d1c4443673b7efcec230
Reviewed-on: https://review.whamcloud.com/31120
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: James Nunez <james.a.nunez@intel.com>
Reviewed-by: Hongchao Zhang <hongchao.zhang@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>