tun: add missing rcu annotation

This patch fixes the following sparse warnings:

drivers/net/tun.c:2241:15: error: incompatible types in comparison expression (different address spaces)

Fixes: cd5681d7 ("tuntap: rename struct tun_steering_prog to struct tun_prog")
Cc: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>