perf kmem: Add kmem.default config option

Currently perf kmem command will select --slab if neither --slab nor
--page is given for backward compatibility.  Add kmem.default config
option to select the default value ('page' or 'slab').

  # cat ~/.perfconfig
  [kmem]
  	default = page

  # perf kmem stat

  SUMMARY (page allocator)
  ========================
  Total allocation requests     :            1,518   [            6,096 KB ]
  Total free requests           :            1,431   [            5,748 KB ]

  Total alloc+freed requests    :            1,330   [            5,344 KB ]
  Total alloc-only requests     :              188   [              752 KB ]
  Total free-only requests      :              101   [              404 KB ]

  Total allocation failures     :                0   [                0 KB ]
  ...

Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Acked-by: Pekka Enberg <penberg@kernel.org>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Joonsoo Kim <js1304@gmail.com>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Taeung Song <treeze.taeung@gmail.com>
Cc: linux-mm@kvack.org
Link: http://lkml.kernel.org/r/1429592107-1807-6-git-send-email-namhyung@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>