refperf: Dynamically allocate experiment-summary output buffer

Currently, the buffer used to accumulate the experiment-summary output
is fixed size, which will cause problems if someone decides to run
one hundred experiments.  This commit therefore dynamically allocates
this buffer.

Cc: Joel Fernandes (Google) <joel@joelfernandes.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>