io-wq: shuffle io_worker_handle_work() code

This is a preparation patch, it adds some helpers and makes
the next patches cleaner.

- extract io_impersonate_work() and io_assign_current_work()
- replace @next label with nested do-while
- move put_work() right after NULL'ing cur_work.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>