perftune.py: PerfTuneBase.compute_cpu_mask_for_mode(): cosmetics: fix a variable name
perftune.py: PerfTuneBase.compute_cpu_mask_for_mode(): cosmetics: fix a variable name

Fix the confusing local variable name - it's a Compute CPU SET what this method
is calculating - not IRQs CPU SET.
The later equals to what this method returns XOR 'cpu_mask'.

scylladb/seastar,scylladb/seastar,syuu1228/seastar,syuu1228/seastar,syuu1228/seastar,scylladb/seastar