If we end up splitting a bio and the queue goes away between
the initial submission and the later split submission, then we
can block forever in blk_queue_enter() waiting for the reference
to drop to zero. This will never happen, since we already hold
a reference.

Mark a split bio as already having entered the queue, so we can
just use the live non-blocking queue enter variant.

Thanks to Tetsuo Handa for the analysis.

Reported-by:  <syzbot+c4f9cebf9d651f6e54de@syzkaller.appspotmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>