bfq: Extract bfq_group_set_weight from bfq_io_set_weight_legacy

This function will be useful when we update weight from the soon-coming
per-device interface.

Signed-off-by: Fam Zheng <zhengfeiran@bytedance.com>
Reviewed-by: Paolo Valente <paolo.valente@linaro.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>