io_uring: drop io_file_put() 'file' argument

Since the fget/fput handling was reworked in commit 09bb8394

, we
never call io_file_put() with state == NULL (and hence file != NULL)
anymore. Remove that case.

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