io_uring/epoll: disallow adding an epoll file to an epoll context

io_uring/epoll: disallow adding an epoll file to an epoll context

One of the nastier things about epoll is how it allows adding epoll
files to epoll contexts. This leads to all sorts of loop detection
code, and has been a source of issues in the past.

Arguably adding IORING_EPOLL_CTL is a historical mistake on the
io_uring side, but we're kind of stuck with it now as it does seem
to be in use according to code searches. But we can at least minimize
the damage a bit and just disallow this part of epoll, where nesting
issues can arise.

Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>