mm: do not shrink pages marked for reclaim by MADV_FREE

MADV_FREE clears pte dirty bit and then marks the page lazyfree (clear
SwapBacked). PPR increments ISOLATE_FILES count, then isolates page and
invokes a reclaim. Inbetween if this lazyfreed page is touched by user then
it becomes dirty.  PPR in shrink_page_list in try_to_unmap finds the page
dirty, marks it back as PageSwapBacked and skips reclaim. As PageSwapBacked
set, PPR identifies the page as anon and decrements ISOLATED_ANON, thus
creating isolated count mismatch.

This results in too_many_isolated() check causing delay in reclaim. Skip
reclaiming lazyfreed pages in PPR path.

Change-Id: I87223c4fa492c5e373ac48f116384b5de03da9fa
Signed-off-by: Prakash Gupta <guptap@codeaurora.org>
Signed-off-by: Vinayak Menon <vinmenon@codeaurora.org>