Impact: save memory

The struct dyn_ftrace table is very large, this patch will save
about 50%.
Signed-off-by: NLai Jiangshan <laijs@cn.fujitsu.com>
Cc: Steven Rostedt <srostedt@redhat.com>
LKML-Reference: <49BA2C9F.8020009@cn.fujitsu.com>
Signed-off-by: NIngo Molnar <mingo@elte.hu>