io_uring/zcrx: init id for xa_find

xa_find() interprets id as the lower bound and thus expects it initialised.

Reported-by: syzbot+c3ff04150c30d3df0f57@syzkaller.appspotmail.com
Fixes: 76f1cc9 ("io_uring/zcrx: add support for multiple ifqs")
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/faea44ef63131e6968f635e1b6b7ca6056f1f533.1748359655.git.asml.silence@gmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

Author: isilence

io_uring/zcrx.c

@@ -630,12 +630,13 @@ int io_register_zcrx_ifq(struct io_ring_ctx *ctx,
 void io_unregister_zcrx_ifqs(struct io_ring_ctx *ctx)
 {
 	struct io_zcrx_ifq *ifq;
-	unsigned long id;
 
 	lockdep_assert_held(&ctx->uring_lock);
 
 	while (1) {
 		scoped_guard(mutex, &ctx->mmap_lock) {
+			unsigned long id = 0;
+
 			ifq = xa_find(&ctx->zcrx_ctxs, &id, ULONG_MAX, XA_PRESENT);
 			if (ifq)
 				xa_erase(&ctx->zcrx_ctxs, id);