refscale: Optimize process_durations()

process_durations() is not a hot path, but there is no good reason to
iterate over and over the data already in 'buf'.

Using a seq_buf saves some useless strcat() and the need of a temp buffer.
Data is written directly at the correct place.

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Tested-by: "Paul E. McKenney" <paulmck@kernel.org>
Reviewed-by: Davidlohr Bueso <dave@stgolabs.net>
Signed-off-by: Neeraj Upadhyay <neeraj.upadhyay@kernel.org>