io_uring: extract io_put_kbuf() helper

Extract a common helper for cleaning up a selected buffer, this will be
used shortly. By the way, correct cflags types to unsigned and, as kbufs
are anyway tracked by a flag, remove useless zeroing req->rw.addr.

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