Convert tres_usage_in_max to kb

As noted by Josko Plazonic in the bug report, the tres_usage_in_max
value is bytes, whereas the script expects kb.

Bug 6004

contribs/seff/seff

@@ -127,7 +127,7 @@ for my $step (@{$job->{'steps'}}) {
     $tot_cpu_usec += $step->{'tot_cpu_usec'};
 
     if (exists $step->{'stats'} && exists $step->{'stats'}{'tres_usage_in_max'}) {
-        my $lmem = Slurmdb::find_tres_count_in_string($step->{'stats'}{'tres_usage_in_max'}, TRES_MEM);
+        my $lmem = Slurmdb::find_tres_count_in_string($step->{'stats'}{'tres_usage_in_max'}, TRES_MEM) / 1024;
 
         if ($mem < $lmem) {
             $mem = $lmem;