perf kmem: Add --sort hit and --sort frag

This patch adds support for "--sort hit" and "--sort frag" to
the "perf kmem" tool. The former was already mentioned in the
help text and the latter is useful for finding call-sites that
exhibit worst case behavior for SLAB allocators.

Signed-off-by: Pekka Enberg <penberg@cs.helsinki.fi>
Cc: Li Zefan <lizf@cn.fujitsu.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Eduard - Gabriel Munteanu <eduard.munteanu@linux360.ro>
Cc: linux-mm@kvack.org <linux-mm@kvack.org>
LKML-Reference: <1258883880-7149-1-git-send-email-penberg@cs.helsinki.fi>
Signed-off-by: Ingo Molnar <mingo@elte.hu>