mm: softdirty: make freshly remapped file pages being softdirty unconditionally

Hugh reported:

 | I noticed your soft_dirty work in install_file_pte(): which looked
 | good at first, until I realized that it's propagating the soft_dirty
 | of a pte it's about to zap completely, to the unrelated entry it's
 | about to insert in its place.  Which seems very odd to me.

Indeed this code ends up being nop in result -- pte_file_mksoft_dirty()
operates with pte_t argument and returns new pte_t which were never used
after.  After looking more I think what we need is to soft-dirtify all
newely remapped file pages because it should look like a new mapping for
memory tracker.
Signed-off-by: NCyrill Gorcunov <gorcunov@openvz.org>
Reported-by: NHugh Dickins <hughd@google.com>
Cc: Pavel Emelyanov <xemul@parallels.com>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

mm/fremap.c

@@ -82,13 +82,10 @@ static int install_file_pte(struct mm_struct *mm, struct vm_area_struct *vma,
 
 	ptfile = pgoff_to_pte(pgoff);
 
-	if (!pte_none(*pte)) {
-		if (pte_present(*pte) && pte_soft_dirty(*pte))
-			pte_file_mksoft_dirty(ptfile);
+	if (!pte_none(*pte))
 		zap_pte(mm, vma, addr, pte);
-	}
 
-	set_pte_at(mm, addr, pte, ptfile);
+	set_pte_at(mm, addr, pte, pte_file_mksoft_dirty(ptfile));
 	/*
 	 * We don't need to run update_mmu_cache() here because the "file pte"
 	 * being installed by install_file_pte() is not a real pte - it's a