dm flakey: fix a bug with 32-bit highmem systems

The function page_address does not work with 32-bit systems with high
memory. Use bvec_kmap_local/kunmap_local instead.

Cc: stable@vger.kernel.org
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Reviewed-by: Sweet Tea Dorminy <sweettea-kernel@dorminy.me>
Signed-off-by: Mike Snitzer <snitzer@kernel.org>