io_uring: deprecate epoll_ctl support

As far as we know, nobody ever adopted the epoll_ctl management via
io_uring. Deprecate it now with a warning, and plan on removing it in
a later kernel version. When we do remove it, we can revert the following
commits as well:

39220e8d4a ("eventpoll: support non-blocking do_epoll_ctl() calls")
58e41a44c4 ("eventpoll: abstract out epoll_ctl() handler")

Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Link: https://lore.kernel.org/io-uring/CAHk-=wiTyisXBgKnVHAGYCNvkmjk=50agS2Uk6nr+n3ssLZg2w@mail.gmail.com/
Signed-off-by: Jens Axboe <axboe@kernel.dk>

io_uring/epoll.c

@@ -26,6 +26,10 @@ int io_epoll_ctl_prep(struct io_kiocb *req, const struct io_uring_sqe *sqe)
 {
 	struct io_epoll *epoll = io_kiocb_to_cmd(req);
 
+	pr_warn_once("%s: epoll_ctl support in io_uring is deprecated and will "
+		     "be removed in a future Linux kernel version.\n",
+		     current->comm);
+
 	if (sqe->buf_index || sqe->splice_fd_in)
 		return -EINVAL;