perf kmem: Show usage if no option is specified

As Ingo suggested, make "perf kmem" show help information.
"perf kmem stat [--caller] [--alloc] .." will show memory
statistics.

Signed-off-by: Li Zefan <lizf@cn.fujitsu.com>
Acked-by: Pekka Enberg <penberg@cs.helsinki.fi>
LKML-Reference: <4B20A195.8030106@cn.fujitsu.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>