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

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

Turns out that my naive assumption that surely nobody is using io_uring
to manage nested epoll instances was wrong - libuv is in fact doing that
very thing. Since we have users in the wild using that feature, we
cannot remove it from upstream. Revert the commit that introduced that
limitation.

This reverts commit cfa1539b24aff18ecb71c6334e7270f810d145bb.

Link: https://lore.kernel.org/io-uring/605939add4dfb674bd7ab39ef8dbd3fd22e754e4.camel@xry111.site/
Reported-by: Xi Ruoyao <xry111@xry111.site>
Signed-off-by: Jens Axboe <axboe@kernel.dk>