io_uring/msg_ring: fix uninitialized use of target_req->flags

syzbot reports that KMSAN complains that 'nr_tw' is an uninit-value
with the following report:

BUG: KMSAN: uninit-value in io_req_local_work_add io_uring/io_uring.c:1192 [inline]
BUG: KMSAN: uninit-value in io_req_task_work_add_remote+0x588/0x5d0 io_uring/io_uring.c:1240
 io_req_local_work_add io_uring/io_uring.c:1192 [inline]
 io_req_task_work_add_remote+0x588/0x5d0 io_uring/io_uring.c:1240
 io_msg_remote_post io_uring/msg_ring.c:102 [inline]
 io_msg_data_remote io_uring/msg_ring.c:133 [inline]
 io_msg_ring_data io_uring/msg_ring.c:152 [inline]
 io_msg_ring+0x1c38/0x1ef0 io_uring/msg_ring.c:305
 io_issue_sqe+0x383/0x22c0 io_uring/io_uring.c:1710
 io_queue_sqe io_uring/io_uring.c:1924 [inline]
 io_submit_sqe io_uring/io_uring.c:2180 [inline]
 io_submit_sqes+0x1259/0x2f20 io_uring/io_uring.c:2295
 __do_sys_io_uring_enter io_uring/io_uring.c:3205 [inline]
 __se_sys_io_uring_enter+0x40c/0x3ca0 io_uring/io_uring.c:3142
 __x64_sys_io_uring_enter+0x11f/0x1a0 io_uring/io_uring.c:3142
 x64_sys_call+0x2d82/0x3c10 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

which is the following check:

if (nr_tw < nr_wait)
	return;

in io_req_local_work_add(). While nr_tw itself cannot be uninitialized,
it does depend on req->flags, which off the msg ring issue path can
indeed be uninitialized.

Fix this by always clearing the allocated 'req' fully if we can't grab
one from the cache itself.

Fixes: 50cf5f38 ("io_uring/msg_ring: add an alloc cache for io_kiocb entries")
Reported-by: syzbot+82609b8937a4458106ca@syzkaller.appspotmail.com
Link: https://lore.kernel.org/io-uring/000000000000fd3d8d061dfc0e4a@google.com/Signed-off-by: Jens Axboe <axboe@kernel.dk>

io_uring/msg_ring.c

@@ -110,10 +110,10 @@ static struct io_kiocb *io_msg_get_kiocb(struct io_ring_ctx *ctx)
 	if (spin_trylock(&ctx->msg_lock)) {
 		req = io_alloc_cache_get(&ctx->msg_cache);
 		spin_unlock(&ctx->msg_lock);
+		if (req)
+			return req;
 	}
-	if (req)
-		return req;
-	return kmem_cache_alloc(req_cachep, GFP_KERNEL | __GFP_NOWARN);
+	return kmem_cache_alloc(req_cachep, GFP_KERNEL | __GFP_NOWARN | __GFP_ZERO);
 }
 
 static int io_msg_data_remote(struct io_kiocb *req)