io_uring: inline io_dismantle_req()

io_dismantle_req() is only used in __io_req_complete_post(), open code
it there.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/ba8f20cb2c914eefa2e7d120a104a198552050db.1687518903.git.asml.silence@gmail.com


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