LU-1538 tests: sanity/101d to check available space

LU-1538 tests: sanity/101d to check available space

Fix the check which compared MBs (in size variable) with KBs
(reported by df).  Also avoid failure if the read took under
two seconds, since the timing is inaccurate at that resolution.

Signed-off-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Change-Id: I43335d699ad2b7e4c5db00c36a7795683f3b04f7
Reviewed-on: http://review.whamcloud.com/4312
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Nathaniel Clark <nathaniel.l.clark@intel.com>
Reviewed-by: Jian Yu <jian.yu@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>