tracepoint: simplification for tracepoints using RCU

Impact: simplify implementation

Now, unused memory is handled by struct tp_probes.

old code use these three field to handle unused memory.
struct tracepoint_entry {
	...
	struct rcu_head rcu;
	void *oldptr;
	unsigned char rcu_pending:1;
	...
};

in this way, unused memory is handled by struct tracepoint_entry.
it bring reenter bug(it was fixed) and tracepoint.c is filled
full of ".*rcu.*" code statements. this patch removes all these.

and:
  rcu_barrier_sched() is removed.
  Do not need regain tracepoints_mutex after tracepoint_update_probes()
  several little cleanup.

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>