nbd: always initialize struct msghdr completely

In the Linux kernel, the following vulnerability has been resolved:

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