Fix NULL bio crash in loop worker thread

After LOOP_SET_FD/LOOP_CLR_FD combo loop device's queue gets request handler which is persistent.

After, say

mount -t iso9660 /dev/loop0 /mnt # sic

this request handler is called directly with
a) ->lo_state being Lo_unbound
b) ->lo_pending being zero

Error path in loop_make_request() completes ->lo_bh_done completion which is persistent as well.

Now, let's start worker thread as usual. It'll set ->lo_pending to 1, don't wait for completion because it was already completed (brokenly), and will not get out of infinite loop because of ->lo_pending. Loop device doesn't have bios at this point and triggers BUG_ON.

So, don't complete ->lo_bh_done when loop device isn't setup fully.

Steps to reproduce:

#!/bin/sh -x
ISO=1.iso
mount -o loop $ISO /mnt
umount /mnt
mount -t iso9660 /dev/loop0 /mnt # sic
mount -o loop $ISO /mnt

---

 drivers/block/loop.c |    3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

@@ -521,7 +521,7 @@ static int loop_make_request(request_queue_t *q, struct bio *old_bio)
 
 	spin_lock_irq(&lo->lo_lock);
 	if (lo->lo_state != Lo_bound)
-		goto out;
+		goto out_not_bound;
 	if (unlikely(rw == WRITE && (lo->lo_flags & LO_FLAGS_READ_ONLY)))
 		goto out;
 	lo->lo_pending++;

@@ -533,6 +533,7 @@ static int loop_make_request(request_queue_t *q, struct bio *old_bio)
 out:
 	if (lo->lo_pending == 0)
 		complete(&lo->lo_bh_done);
+out_not_bound:
 	spin_unlock_irq(&lo->lo_lock);
 	bio_io_error(old_bio, old_bio->bi_size);
 	return 0;