block: assign correct tag before doing prefetch of request

Ensure that current tag is correctly assigned before attempting
to prefetch the first cacheline of the request.

Fixes: 92aff191 ("block: prefetch request to be initialized")
Reported-and-tested-by:  <syzbot+cd20829ac44b92bf6ed0@syzkaller.appspotmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>