fsperf: Adjust some titiles for plot

Signed-off-by: OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>
fsperf: Remove "%lf %lf" format for gnuplot

It looks like gnuplot-4.6.0 doesn't work "%lf %lf". And "%lf %lf" itself
seems to be unnecessary.

So, remove it unless seeing the problem.

Signed-off-by: OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>