block, bfq: clarify the goal of bfq_split_bfqq()

The exact, general goal of the function bfq_split_bfqq() is not that
apparent. Add a comment to make it clear.

Tested-by: Oleksandr Natalenko <oleksandr@natalenko.name>
Signed-off-by: Paolo Valente <paolo.valente@linaro.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>