mm: trace-vmscan-postprocess.pl: report the number of file/anon pages respectively

Until now, the reporting from trace-vmscan-postprocess.pl is not very
useful because we cannot directly use this script for checking the
file/anon ratio of scanning.  This patch aims to report respectively the
number of file/anon pages which were scanned/reclaimed by kswapd or
direct-reclaim.  Sample output is usually something like the following.

Summary
Direct reclaims:                          8823
Direct reclaim pages scanned:             2438797
Direct reclaim file pages scanned:        1315200
Direct reclaim anon pages scanned:        1123597
Direct reclaim pages reclaimed:           446139
Direct reclaim file pages reclaimed:      378668
Direct reclaim anon pages reclaimed:      67471
Direct reclaim write file sync I/O:       0
Direct reclaim write anon sync I/O:       0
Direct reclaim write file async I/O:      0
Direct reclaim write anon async I/O:      4240
Wake kswapd requests:                     122310
Time st...