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: I74adf5ba6add7024ce7973d2850c3cb13f4da0cb
Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-on: http://review.whamcloud.com/941
Reviewed-by: Yu Jian <yujian@whamcloud.com>
Tested-by: Hudson
Reviewed-by: Niu Yawei <niu@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>