io_uring: fix mshot read defer taskrun cqe posting

BugLink: https://bugs.launchpad.net/bugs/2060531

[ Upstream commit 70581dcd0601a09f134f23268e3374e15d736824 ]

We can't post CQEs from io-wq with DEFER_TASKRUN set, normal completions
are handled but aux should be explicitly disallowed by opcode handlers.

Cc: stable@vger.kernel.org
Fixes: fc68fcda04 ("io_uring/rw: add support for IORING_OP_READ_MULTISHOT")
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/6fb7cba6f5366da25f4d3eb95273f062309d97fa.1709740837.git.asml.silence@gmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Paolo Pisati <paolo.pisati@canonical.com>

io_uring/rw.c

@@ -932,6 +932,8 @@ int io_read_mshot(struct io_kiocb *req, unsigned int issue_flags)
 	 */
 	if (!file_can_poll(req->file))
 		return -EBADFD;
+	if (issue_flags & IO_URING_F_IOWQ)
+		return -EAGAIN;
 
 	ret = __io_read(req, issue_flags);