time_bench_sample: change result semantics a bit

Report the cost of e.g. (spin_lock + spin_unlock) instead of
dividing cost by two as before.  One sync-primitive does not go
without the other.

Results listed below for a E5-2695 CPU:

Type:for_loop Per elem: 0 cycles(tsc) 0.359 ns
 - (measurement period time:0.359908674 sec time_interval:359908674)
 - (invoke count:1000000000 tsc_interval:899770659)

Type:spin_lock_unlock Per elem: 41 cycles(tsc) 16.449 ns
 - (measurement period time:1.644912922 sec time_interval:1644912922)
 - (invoke count:100000000 tsc_interval:4112279678)

Type:local_BH_disable_enable Per elem: 18 cycles(tsc) 7.459 ns
 - (measurement period time:0.745900804 sec time_interval:745900804)
 - (invoke count:100000000 tsc_interval:1864750644)

Type:local_IRQ_disable_enable Per elem: 7 cycles(tsc) 2.860 ns
 - (measurement period time:0.286085358 sec time_interval:286085358)
 - (invoke count:100000000 tsc_interval:715212653)

Type:local_irq_save_restore Per elem: 37 cycles(tsc) 14.840 ns
 - (measurement period time:1.484042176 sec time_interval:1484042176)
 - (invoke count:100000000 tsc_interval:3710103509)

Type:preempt_disable_enable Per elem: 10 cycles(tsc) 4.291 ns
 - (measurement period time:0.429124194 sec time_interval:429124194)
 - (invoke count:100000000 tsc_interval:1072809603)

Type:funcion_call_cost Per elem: 5 cycles(tsc) 2.145 ns
 - (measurement period time:0.214560396 sec time_interval:214560396)
 - (invoke count:100000000 tsc_interval:536400278)

Type:func_ptr_call_cost Per elem: 6 cycles(tsc) 2.503 ns
 - (measurement period time:0.250320522 sec time_interval:250320522)
 - (invoke count:100000000 tsc_interval:625800572)

Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>