block: fix too broad elevator check in blk_mq_free_request()

We added RQF_ELV to tell whether there's an IO scheduler attached, and
RQF_ELVPRIV tells us whether there's an IO scheduler with private data
attached. Don't check RQF_ELV in blk_mq_free_request(), what we care
about here is just if we have scheduler private data attached.

This fixes a boot crash

Fixes: 2ff0682d

 ("block: store elevator state in request")
Reported-by: Yi Zhang <yi.zhang@redhat.com>
Reported-by:  <syzbot+eb8104072aeab6cc1195@syzkaller.appspotmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>