LU-3300 lprocfs: interpret result of dt_statfs() correctly

LU-3300 lprocfs: interpret result of dt_statfs() correctly

In a7369bcd860af61a073cb2424139e3acccdcb203, 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.

Signed-off-by: John L. Hammond <john.hammond@intel.com>
Change-Id: I91873f23cd69c2217474c26b9751aec259fae155
Reviewed-on: http://review.whamcloud.com/6385
Reviewed-by: Robert Read <robert.read@intel.com>
Tested-by: Hudson
Reviewed-by: Emoly Liu <emoly.liu@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>