tools/vm/page_owner_sort.c: filter out unneeded line

To see a sorted result from page_owner, there must be a tiresome
preprocessing step before running page_owner_sort.  This patch simply
filters out lines which start with "PFN" while reading the page owner
report.

Signed-off-by: Changhee Han <ch0.han@lge.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Link: http://lkml.kernel.org/r/20200429052940.16968-1-ch0.han@lge.com


Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>