io_uring/register: don't get a reference to the registered ring fd

io_uring/register: don't get a reference to the registered ring fd

This isn't necessary and was only done because the register path isn't a
hot path and hence the extra ref/put doesn't matter, and to have the
exit path be able to unconditionally put whatever file was gotten
regardless of the type.

In preparation for sharing this code with the main io_uring_enter(2)
syscall, drop the reference and have the caller conditionally put the
file if it was a normal file descriptor.

Signed-off-by: Jens Axboe <axboe@kernel.dk>