io_uring/tctx: avoid modifying loop variable in io_ring_add_registered_file

io_uring/tctx: avoid modifying loop variable in io_ring_add_registered_file

Use a separate 'idx' variable to store the result of array_index_nospec()
instead of modifying the loop variable 'offset' directly. This improves
code clarity by separating the logical index from the sanitized index
used for array access.

No functional change intended.

Signed-off-by: Yang Xiuwei <yangxiuwei@kylinos.cn>
Signed-off-by: Jens Axboe <axboe@kernel.dk>