nbd: always initialize struct msghdr completely

syzbot complains that msg->msg_get_inq value can be uninitialized [1]

struct msghdr got many new fields recently, we should always make
sure their values is zero by default.

[1]
 BUG: KMSAN: uninit-value in tcp_recvmsg+0x686/0xac0 net/ipv4/tcp.c:2571
  tcp_recvmsg+0x686/0xac0 net/ipv4/tcp.c:2571
  inet_recvmsg+0x131/0x580 net/ipv4/af_inet.c:879
  sock_recvmsg_nosec net/socket.c:1044 [inline]
  sock_recvmsg+0x12b/0x1e0 net/socket.c:1066
  __sock_xmit+0x236/0x5c0 drivers/block/nbd.c:538
  nbd_read_reply drivers/block/nbd.c:732 [inline]
  recv_work+0x262/0x3100 drivers/block/nbd.c:863
  process_one_work kernel/workqueue.c:2627 [inline]
  process_scheduled_works+0x104e/0x1e70 kernel/workqueue.c:2700
  worker_thread+0xf45/0x1490 kernel/workqueue.c:2781
  kthread+0x3ed/0x540 kernel/kthread.c:388
  ret_from_fork+0x66/0x80 arch/x86/kernel/process.c:147
  ret_from_fork_asm+0x11/0x20 arch/x86/entry/entry_64.S:242

Local variable msg created at:
  __sock_xmit+0x4c/0x5c0 drivers/block/nbd.c:513
  nbd_read_reply drivers/block/nbd.c:732 [inline]
  recv_work+0x262/0x3100 drivers/block/nbd.c:863

CPU: 1 PID: 7465 Comm: kworker/u5:1 Not tainted 6.7.0-rc7-syzkaller-00041-gf016f7547aee #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 11/17/2023
Workqueue: nbd5-recv recv_work

Fixes: f94fd25 ("tcp: pass back data left in socket after receive")
Reported-by: syzbot <syzkaller@googlegroups.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: stable@vger.kernel.org
Cc: Josef Bacik <josef@toxicpanda.com>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: linux-block@vger.kernel.org
Cc: nbd@other.debian.org
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://lore.kernel.org/r/20240112132657.647112-1-edumazet@google.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

Author: edumazet

drivers/block/nbd.c

@@ -508,7 +508,7 @@ static int __sock_xmit(struct nbd_device *nbd, struct socket *sock, int send,
 		       struct iov_iter *iter, int msg_flags, int *sent)
 {
 	int result;
-	struct msghdr msg;
+	struct msghdr msg = {} ;
 	unsigned int noreclaim_flag;
 
 	if (unlikely(!sock)) {
@@ -524,10 +524,6 @@ static int __sock_xmit(struct nbd_device *nbd, struct socket *sock, int send,
 	do {
 		sock->sk->sk_allocation = GFP_NOIO | __GFP_MEMALLOC;
 		sock->sk->sk_use_task_frag = false;
-		msg.msg_name = NULL;
-		msg.msg_namelen = 0;
-		msg.msg_control = NULL;
-		msg.msg_controllen = 0;
 		msg.msg_flags = msg_flags | MSG_NOSIGNAL;
 
 		if (send)