ovs-benchmark: improve 'rate' command piped output support.

ovs-benchmark: improve 'rate' command piped output support.

Add fflush() to the 'rate' command of 'ovs-benchmark'.  On a tty console
the output is line-buffered. However, piped output by default has a
deeper (usually 4KB) buffer, the output won't appear to the other end of
the pipe until this buffer is full.

This patch makes the piped output of the 'rate' command's consistent
with tty console output.

Signed-off-by: Andy Zhou <azhou@nicira.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>