mm/page_alloc: Call kernel_map_pages in unset_migrateype_isolate

Commit d1037ba0b8 (mm/page_alloc: restrict max order of
merging on isolated pageblock) changed the logic of unset_migratetype_isolate
to check the buddy allocator and explicitly call __free_pages to
merge. The page that is being freed in this path never had prep_new_page
called so set_page_refcounted is called explicitly but there is
no call to kernel_map_pages. With the default kernel_map_pages this
is mostly harmless but if kernel_map_pages does any manipulation
of the page tables (unmapping or setting pages to read only) this
may trigger a fault:

    alloc_contig_range test_pages_isolated(ceb00, ced00) failed
    Unable to handle kernel paging request at virtual address ffffffc0cec00000
    pgd = ffffffc045fc4000
    [ffffffc0cec00000] *pgd=0000000000000000
    Internal error: Oops: 9600004f [#1] PREEMPT SMP
    Modules linked in: exfatfs
    CPU: 1 PID: 23237 Comm: TimedEventQueue Not tainted 3.10.49-gc72ad36-dirty #1
    task: ffffffc03de52100 ti: ffffffc015388000 task.ti: ffffffc015388000
    PC is at memset+0xc8/0x1c0
    LR is at kernel_map_pages+0x1ec/0x244

Fix this by calling kernel_map_pages to ensure the page is set in the
page table properly

Change-Id: Ie0c7f38fce24683b6ddebf95874be662ef25021b
Signed-off-by: Laura Abbott <lauraa@codeaurora.org>

mm/page_isolation.c

@@ -102,6 +102,7 @@ void unset_migratetype_isolate(struct page *page, unsigned migratetype)
 
 			if (!is_migrate_isolate_page(buddy)) {
 				__isolate_free_page(page, order);
+				kernel_map_pages(page, (1 << order), 1);
 				set_page_refcounted(page);
 				isolated_page = page;
 			}