ksm: cond_resched in unstable tree

KSM needs a cond_resched() for CONFIG_PREEMPT_NONE, in its unbounded
search of the unstable tree.  The stable tree cases already have one,
and originally there was one down inside get_user_pages();
but I missed it when I converted to follow_page() instead.
Signed-off-by: Hugh Dickins <hugh.dickins@tiscali.co.uk>
Acked-by: Izik Eidus <ieidus@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/ksm.c

@@ -1012,6 +1012,7 @@ static struct rmap_item *unstable_tree_search_insert(struct page *page,
 		struct rmap_item *tree_rmap_item;
 		int ret;
 
+		cond_resched();
 		tree_rmap_item = rb_entry(*new, struct rmap_item, node);
 		page2[0] = get_mergeable_page(tree_rmap_item);
 		if (!page2[0])