io_uring/net: always initialize kmsg->msg.msg_inq upfront

io_uring/net: always initialize kmsg->msg.msg_inq upfront

syzbot reports that ->msg_inq may get used uinitialized from the
following path:

BUG: KMSAN: uninit-value in io_recv_buf_select io_uring/net.c:1094 [inline]
BUG: KMSAN: uninit-value in io_recv+0x930/0x1f90 io_uring/net.c:1158
 io_recv_buf_select io_uring/net.c:1094 [inline]
 io_recv+0x930/0x1f90 io_uring/net.c:1158
 io_issue_sqe+0x420/0x2130 io_uring/io_uring.c:1740
 io_queue_sqe io_uring/io_uring.c:1950 [inline]
 io_req_task_submit+0xfa/0x1d0 io_uring/io_uring.c:1374
 io_handle_tw_list+0x55f/0x5c0 io_uring/io_uring.c:1057
 tctx_task_work_run+0x109/0x3e0 io_uring/io_uring.c:1121
 tctx_task_work+0x6d/0xc0 io_uring/io_uring.c:1139
 task_work_run+0x268/0x310 kernel/task_work.c:239
 io_run_task_work+0x43a/0x4a0 io_uring/io_uring.h:343
 io_cqring_wait io_uring/io_uring.c:2527 [inline]
 __do_sys_io_uring_enter io_uring/io_uring.c:3439 [inline]
 __se_sys_io_uring_enter+0x204f/0x4ce0 io_uring/io_uring.c:3330
 __x64_sys_io_uring_enter+0x11f/0x1a0 io_uring/io_uring.c:3330
 x64_sys_call+0xce5/0x3c30 arch/x86/include/generated/asm/syscalls_64.h:427
 do_syscall_x64 arch/x86/entry/common.c:52 [inline]
 do_syscall_64+0xcd/0x1e0 arch/x86/entry/common.c:83
 entry_SYSCALL_64_after_hwframe+0x77/0x7f

and it is correct, as it's never initialized upfront. Hence the first
submission can end up using it uninitialized, if the recv wasn't
successful and the networking stack didn't honor ->msg_get_inq being set
and filling in the output value of ->msg_inq as requested.

Set it to 0 upfront when it's allocated, just to silence this KMSAN
warning. There's no side effect of using it uninitialized, it'll just
potentially cause the next receive to use a recv value hint that's not
accurate.

Fixes: c6f32c7d9e09 ("io_uring/net: get rid of ->prep_async() for receive side")
Reported-by: syzbot+068ff190354d2f74892f@syzkaller.appspotmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>