ion: use vm_insert_pfn for faulted pages

Most ion userspace mappings are created with remap_pfn_range.  Use
vm_insert_pfn instead of vm_insert_page to make faulted cached
mappings look more like uncached mappings.

Signed-off-by: Colin Cross <ccross@android.com>
Signed-off-by: John Stultz <john.stultz@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>