io_uring/kbuf: align legacy buffer add limit with MAX_BIDS_PER_BGID

io_uring/kbuf: align legacy buffer add limit with MAX_BIDS_PER_BGID

io_provide_buffers_prep() accepts nbufs up to MAX_BIDS_PER_BGID, but
io_add_buffers() stops when bl->nbufs reaches USHRT_MAX. This makes the
effective add limit one lower than the validated limit.

Use MAX_BIDS_PER_BGID in the add-side boundary check so validation and
execution use the same limit, and update the comment to refer to the
actual limit constant.

Signed-off-by: liyouhong <liyouhong@kylinos.cn>
Link: https://patch.msgid.link/20260528024936.3672659-1-dayou5941@163.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>