tun: add missing rcu annotation in tun_set_ebpf()

We expecte prog_p to be protected by rcu, so adding the rcu annotation
to fix the following sparse warning:

drivers/net/tun.c:3003:36: warning: incorrect type in argument 2 (different address spaces)
drivers/net/tun.c:3003:36:    expected struct tun_prog [noderef] __rcu **prog_p
drivers/net/tun.c:3003:36:    got struct tun_prog **prog_p
drivers/net/tun.c:3292:42: warning: incorrect type in argument 2 (different address spaces)
drivers/net/tun.c:3292:42:    expected struct tun_prog **prog_p
drivers/net/tun.c:3292:42:    got struct tun_prog [noderef] __rcu **
drivers/net/tun.c:3296:42: warning: incorrect type in argument 2 (different address spaces)
drivers/net/tun.c:3296:42:    expected struct tun_prog **prog_p
drivers/net/tun.c:3296:42:    got struct tun_prog [noderef] __rcu **

Reported-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/tun.c

@@ -2983,7 +2983,7 @@ unlock:
 	return ret;
 }
 
-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,
 			void __user *data)
 {
 	struct bpf_prog *prog;