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 openzfs#12916 
Closes openzfs#13767

cmd/arcstat.in

@@ -271,7 +271,7 @@ def print_values():
     if pretty_print:
         fmt = lambda col: prettynum(cols[col][0], cols[col][1], v[col])
     else:
-        fmt = lambda col: v[col]
+        fmt = lambda col: str(v[col])
 
     sys.stdout.write(sep.join(fmt(col) for col in hdr))
     sys.stdout.write("\n")