io_uring/zcrx: fix overshooting recv limit

io_uring/zcrx: fix overshooting recv limit

It's reported that sometimes a zcrx request can receive more than was
requested. It's caused by io_zcrx_recv_skb() adjusting desc->count for
all received buffers including frag lists, but then doing recursive
calls to process frag list skbs, which leads to desc->count double
accounting and underflow.

Reported-and-tested-by: Matthias Jasny <matthiasjasny@gmail.com>
Fixes: 6699ec9a23f85 ("io_uring/zcrx: add a read limit to recvzc requests")
Cc: stable@vger.kernel.org
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>