io_uring: set table->files[i] to NULL when io_sqe_file_register failed

While io_sqe_file_register() failed in __io_sqe_files_update(),
table->files[i] still point to the original file which may freed
soon, and that will trigger use-after-free problems.

Cc: stable@vger.kernel.org
Fixes: f3bd9dae

 ("io_uring: fix memleak in __io_sqe_files_update()")
Signed-off-by: Jiufei Xue <jiufei.xue@linux.alibaba.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>