xdp: xdp_umem: fix umem pages mapping for 32bits systems

Use kmap instead of page_address as it's not always in low memory.

Acked-by: Björn Töpel <bjorn.topel@intel.com>
Signed-off-by: Ivan Khoronzhuk <ivan.khoronzhuk@linaro.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>