mm: oom analysis: add buffer cache information to show_free_areas()

It is often useful to know the statistics for all pages that are handled
like page cache pages when looking at OOM log output.

Therefore show_free_areas() should also display buffer cache statistics.

Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Acked-by: Wu Fengguang <fengguang.wu@intel.com>
Reviewed-by: Rik van Riel <riel@redhat.com>
Cc: David Rientjes <rientjes@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>