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: NJason Wang <jasowang@redhat.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/tun.c

@@ -2225,7 +2225,8 @@ static void tun_prog_free(struct rcu_head *rcu)
 	kfree(prog);
 }
 
-static int __tun_set_ebpf(struct tun_struct *tun, struct tun_prog **prog_p,
+static int __tun_set_ebpf(struct tun_struct *tun,
+			  struct tun_prog __rcu **prog_p,
 			  struct bpf_prog *prog)
 {
 	struct tun_prog *old, *new = NULL;