LU-413 limit used inodes for performance tests

LU-413 limit used inodes for performance tests

If doing wide stripe mdsrate tests, don't try to consume all of
the inodes on the OSTs.  The MDS does not consume inodes on OSTs
completely uniformly, and this only introduces spurious failures
when the inodes run out unevenly.

Change-Id: I36243e0d3e990858d268d043012af40419936fe4
Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-on: http://review.whamcloud.com/942
Reviewed-by: Yu Jian <yujian@whamcloud.com>
Reviewed-by: Niu Yawei <yawei.niu@gmail.com>
Tested-by: Hudson
Reviewed-by: Johann Lombardi <johann@whamcloud.com>