mm/page_alloc.c: call kernel_map_pages in unset_migrateype_isolate

Commit 3c605096 ("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: ...