io_uring: make OP_CLOSE consistent with direct open

From recently open/accept are now able to manipulate fixed file table,
but it's inconsistent that close can't. Close the gap, keep API same as
with open/accept, i.e. via sqe->file_slot.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>