io_uring: use rcu_dereference in io_close

Accessing the file table needs a rcu_dereference_protected().

Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20220518084005.3255380-7-hch@lst.de


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