LU-8586 test: Fix failure due to mixed OST size.

LU-8586 test: Fix failure due to mixed OST size.

Test(pios_ssf) was failing intermittently, because during the
setup, OST's were created with different size. This leads to
smaller OST filling up much faster during the pios run. Once
an OST's is full any subsequent write to FS returns ENOSPC.

This patch modifies function space_check() to determine
available space information by calculating
smallest size "OST x OSTCOUNT". Earlier it was returning
"summary" information as reported by "lfs df".

Test-Parameters: trivial testlist=sanity-benchmark
Signed-off-by: Arshad Hussain <arshad.hussain@seagate.com>
Change-Id: Icec25e9857e5ba45f6bec1265c5697ecd92a7751
Seagate-bug-id: MRP-3243
Reviewed-by: Rahul Deshmukh <rahul.deshmukh@seagate.com>
Reviewed-by: Ashish Purkar <ashish.purkar@seagate.com>
Reviewed-by: Vladimir Saveliev <vladimir.saveliev@seagate.com>
Tested-by: Elena V. Gryaznova <elena.gryaznova@seagate.com>
Reviewed-on: https://review.whamcloud.com/22343
Tested-by: Maloo <hpdd-maloo@intel.com>
Tested-by: Jenkins
Reviewed-by: James Nunez <james.a.nunez@intel.com>
Reviewed-by: Elena Gryaznova <elena.gryaznova@seagate.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>