net-tun: Add type safety to tun_xdp_to_ptr() and tun_ptr_to_xdp()

This reduces likelihood of incorrect use.

Test: builds

Signed-off-by: Maciej Żenczykowski <maze@google.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20200819010710.3959310-1-zenczykowski@gmail.com