LU-5473 tests: sanity/51b Account for ZFS inode size

LU-5473 tests: sanity/51b Account for ZFS inode size

Account for inode size for 11KB for ZFS (and 4KB for LDISKFS) when
checking space on MDC during sanity test_51b.
Do lfs df and lfs df -i on completion of sanity/test_51b.  This helps
determine the correct inode size accounting.

Test-Parameters: trivial mdsfilesystemtype=zfs mdtfilesystemtype=zfs ostfilesystemtype=zfs testlist=sanity envdefinitions=SLOW=yes mdscount=1
Signed-off-by: Nathaniel Clark <nathaniel.l.clark@intel.com>
Change-Id: Icd8e0d55a89d8e3d22bbb1b2ff206e238a7262ac
Reviewed-on: http://review.whamcloud.com/21821
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: James Nunez <james.a.nunez@intel.com>
Reviewed-by: Jian Yu <jian.yu@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>