arcstat: fix -p option

When the -p option is used, a list of floats is passed to sep.join(),
which expects strings. Fix this by converting each value to a string.

Reviewed-by: Richard Elling <Richard.Elling@RichardElling.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Roberto Ricci <ricci@disroot.org>
Closes #12916
Closes #13767