io_uring: disallow close of ring itself

A previous commit enabled this functionality, which also enabled O_PATH
to work correctly with io_uring. But we can't safely close the ring
itself, as the file handle isn't reference counted inside
io_uring_enter(). Instead of jumping through hoops to enable ring
closure, add a "soft" ->needs_file option, ->needs_file_no_error. This
enables O_PATH file descriptors to work, but still catches the case of
trying to close the ring itself.

Reported-by: Jann Horn <jannh@google.com>
Fixes: 904fbcb1 ("io_uring: remove 'fd is io_uring' from close path")
Signed-off-by: Jens Axboe <axboe@kernel.dk>