io_uring: separate the io_free_req and io_free_req_find_next interface

Similar to the distinction between io_put_req and io_put_req_find_next,
io_free_req has been modified similarly, with no functional changes.

Signed-off-by: Jackie Liu <liuyun01@kylinos.cn>
Signed-off-by: Jens Axboe <axboe@kernel.dk>