bfq-iosched: don't call bfqg_and_blkg_put for !CONFIG_BFQ_GROUP_IOSCHED

It's not available if we don't have group io scheduling set, and
there's no need to call it.

Fixes: 0d52af59 ("block, bfq: release oom-queue ref to root group on exit")
Signed-off-by: Jens Axboe <axboe@kernel.dk>