Seeing negative values for wlentime and rlentime

Linux kstat IO and TIMER printed values as signed. However the counters
only increment. Thus humans looking at the data can be confused when
the counters roll over.

Note: The recommended use of these values is to monitor the derivative,
which don't really care about the sign. See explanations related to
non-negative derivatives in the various time-series databases.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Giuseppe Di Natale <guss80@gmail.com>
Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Signed-off-by: Richard Elling <Richard.Elling@RichardElling.com>
Closes #8131
Closes #8198