io_uring: remove 'fd is io_uring' from close path

io_uring: remove 'fd is io_uring' from close path

The attempt protecting us from closing the ring itself wasn't really
complete, and we actually don't need it. The referencing of requests
themselve, and the references they hold on the ring, ensures that the
life time of the ring is sane. With the check removed, we can also
remove the need to have the close operation fget() the file.

Reported-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Jens Axboe <axboe@kernel.dk>