io_uring: use io_file_from_index in io_msg_grab_file

Use io_file_from_index instead of open coding it.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20230620113235.920399-8-hch@lst.de


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