mm: code cleanup for MADV_FREE

mm: code cleanup for MADV_FREE

Some comments for MADV_FREE is revised and added to help people understand
the MADV_FREE code, especially the page flag, PG_swapbacked.  This makes
page_is_file_cache() isn't consistent with its comments.  So the function
is renamed to page_is_file_lru() to make them consistent again.  All these
are put in one patch as one logical change.

Suggested-by: David Hildenbrand <[email protected]>
Suggested-by: Johannes Weiner <[email protected]>
Suggested-by: David Rientjes <[email protected]>
Signed-off-by: "Huang, Ying" <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Acked-by: Johannes Weiner <[email protected]>
Acked-by: David Rientjes <[email protected]>
Acked-by: Michal Hocko <[email protected]>
Acked-by: Pankaj Gupta <[email protected]>
Acked-by: Vlastimil Babka <[email protected]>
Cc: Dave Hansen <[email protected]>
Cc: Mel Gorman <[email protected]>
Cc: Minchan Kim <[email protected]>
Cc: Hugh Dickins <[email protected]>
Cc: Rik van Riel <[email protected]>
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Linus Torvalds <[email protected]>