net-tun: Eliminate two tun/xdp related function calls from vhost-net

This provides a minor performance boost by virtue of inlining
instead of cross module function calls.

Test: builds

Signed-off-by: Maciej Żenczykowski <maze@google.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20200819010710.3959310-2-zenczykowski@gmail.com