mm/madvise: fix madvise_pageout for private file mappings

When MADV_PAGEOUT is called on a private file mapping VMA region, we bail
out early if the process is neither owner nor write capable of the file. 
However, this VMA may have both private/shared clean pages and private
dirty pages.  The opportunity of paging out the private dirty pages (Anon
pages) is missed.  Fix this behavior by allowing private file mappings
pageout further and perform the file access check along with PageAnon()
during page walk.

We observe ~10% improvement in zram usage, thus leaving more available
memory on a 4GB RAM system running Android.

[[email protected]: v2]
  Link: https://lkml.kernel.org/r/[email protected]
Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Pavankumar Kondeti <[email protected]>
Cc: Charan Teja Kalla <[email protected]>
Cc: Minchan Kim <[email protected]>
Cc: Suren Baghdasaryan <[email protected]>
Cc: David Hildenbrand <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

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