lockdep: Fix file mode of lock_stat

/proc/lock_stat is writable.

Signed-off-by: Li Zefan <lizf@cn.fujitsu.com>
Cc: Peter Zijlstra <peterz@infradead.org>
LKML-Reference: <4A7BE7B6.10904@cn.fujitsu.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>