mm, madvise_inject_error: Disable MADV_SOFT_OFFLINE for ZONE_DEVICE pages

Given that dax / device-mapped pages are never subject to page
allocations remove them from consideration by the soft-offline
mechanism.

Reported-by: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
Acked-by: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
Signed-off-by: Dave Jiang <dave.jiang@intel.com>