markers: simplify marker_set_format()

current marker_set_format() is complex this patch simplify it,
and decrease the overhead of marker_update_probes().

Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Acked-by: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
Signed-off-by: Ingo Molnar <mingo@elte.hu>