time_bench_sample: Recording the effect of using a PREEMPT-able kernel.

Be careful not to enable kernel CONFIG_DEBUG_PREEMPT=y when doing
micro-benchmarking.

Effect of enabling kernel CONFIG_PREEMPT=y is almost none, BUT
the config DEBUG_PREEMPT=y made a big difference.

The cost for preempt_{disable,enable} with DEBUG_PREEMPT=y
increased with +8.003 ns from 4.291 ns to 12.294 ns.

Most surprisingly the bottom-half local_BH_{disable,enable} cost
increased significantly with +14.232 ns from 7.459ns to 21.691ns.

A little surprisingly the spinlock cost also increased with +5.728 ns
from 16.449 ns to 22.177 ns (DEBUG_PREEMPT=y).

Print a warning if CONFIG_DEBUG_PREEMPT is enabled.

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