dm kcopyd: use __GFP_HIGHMEM when allocating pages

dm-kcopyd doesn't access the allocated pages directly, it only passes
them to dm-io which adds them to a bio list - thus, we can allocate
the pages from high memory. This will reduce pressure on the low
memory when there are a large number of kcopyd jobs in progress.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@kernel.org>