staging/lustre/lprocfs: interpret result of dt_statfs() correctly

I accidentally reversed the sense of the error check after the call to
dt_statfs() in lprocfs_dt_rd_{blksize,{files,kbytes}{free,avail}.
Unreverse the error checking.

Intel-bug-id: https://jira.hpdd.intel.com/browse/LU-3300
Lustre-change: http://review.whamcloud.com/6385


Signed-off-by: John L. Hammond <john.hammond@intel.com>
Reviewed-by: Robert Read <robert.read@intel.com>
Reviewed-by: Emoly Liu <emoly.liu@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Signed-off-by: Peng Tao <tao.peng@emc.com>
Signed-off-by: Andreas Dilger <andreas.dilger@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>