Revert "tun: remove useless codes in tun_chr_aio_read() and tun_recvmsg()"

This reverts commit 73713357.

MSG_TRUNC handling was broken and is going to be fixed in
the 'net' tree, so revert this.
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/tun.c

@@ -1354,6 +1354,7 @@ static ssize_t tun_chr_aio_read(struct kiocb *iocb, const struct iovec *iv,
 
 	ret = tun_do_read(tun, tfile, iv, len,
 			  file->f_flags & O_NONBLOCK);
+	ret = min_t(ssize_t, ret, len);
 	if (ret > 0)
 		iocb->ki_pos = ret;
 out:
@@ -1454,6 +1455,10 @@ static int tun_recvmsg(struct kiocb *iocb, struct socket *sock,
 	}
 	ret = tun_do_read(tun, tfile, m->msg_iov, total_len,
 			  flags & MSG_DONTWAIT);
+	if (ret > total_len) {
+		m->msg_flags |= MSG_TRUNC;
+		ret = flags & MSG_TRUNC ? ret : total_len;
+	}
 out:
 	tun_put(tun);
 	return ret;