flux-ping: use optparse_get_size_int() for --pad option

Problem: The flux-ping(1) --pad=N option doesn't take an optional
suffix to scale the result, so specification of large pad sizes is
unnecssarily inconvenient.

Use optparse_get_size_int() to access the --pad option argument so
it supports suffixes [kKMG].

Author: grondo

src/cmd/flux-ping.c

@@ -316,9 +316,7 @@ int main (int argc, char *argv[])
     if (!(target = strdup (argv[optindex])))
         log_msg_exit ("out of memory");
 
-    pad_bytes = optparse_get_int (opts, "pad", 0);
-    if (pad_bytes < 0)
-        log_msg_exit ("pad must be >= 0");
+    pad_bytes = optparse_get_size_int (opts, "pad", "0");
 
     ctx.nodeid = FLUX_NODEID_ANY;
     if (optparse_hasopt (opts, "rank")) {