tuntap: set SOCK_ZEROCOPY flag during open

Commit 54f968d6
(tuntap: move socket to tun_file) forgets to set SOCK_ZEROCOPY flag, which will
prevent vhost_net from doing zercopy w/ tap. This patch fixes this by setting
it during file open.

Cc: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: NJason Wang <jasowang@redhat.com>
Acked-by: NMichael S. Tsirkin <mst@redhat.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/tun.c

@@ -2159,6 +2159,8 @@ static int tun_chr_open(struct inode *inode, struct file * file)
 	set_bit(SOCK_EXTERNALLY_ALLOCATED, &tfile->socket.flags);
 	INIT_LIST_HEAD(&tfile->next);
 
+	sock_set_flag(&tfile->sk, SOCK_ZEROCOPY);
+
 	return 0;
 }