perf/sched: Fix 'perf sched trace'

perf/sched: Fix 'perf sched trace'

If we use 'perf sched trace', it will call symbol__init() again,
and can lead to a perf tool crash:

 [root@localhost perf]# ./perf sched trace
 *** glibc detected *** ./perf: free(): invalid next size (normal): 0x094c1898 ***
 ======= Backtrace: =========
 /lib/libc.so.6[0xb7602404]
 /lib/libc.so.6(cfree+0x96)[0xb76043b6]
 ./perf[0x80730fe]
 ./perf[0x8074c97]
 ./perf[0x805eb59]
 ./perf[0x80536fd]
 ./perf[0x804b618]
 ./perf[0x804bdc3]
 /lib/libc.so.6(__libc_start_main+0xe5)[0xb75a9735]
 ./perf[0x804af81]
 ======= Memory map: ========
 08048000-08158000 r-xp 00000000 fe:00 556831     /home/eric/....
 08158000-08168000 rw-p 0010f000 fe:00 556831     /home/eric/...
 08168000-085fe000 rw-p 00000000 00:00 0
 094ab000-094cc000 rw-p 00000000 00:00 0          [heap]

Signed-off-by: Xiao Guangrong <xiaoguangrong@cn.fujitsu.com>
LKML-Reference: <4B1C7EE1.8030906@cn.fujitsu.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>