block: kill deprecated BUG_ON() in the flush handling

We've never had any useful reports from this BUG_ON(), and in fact a
number of the BUG_ON()'s in the flush handling need to be turned into
more graceful handling.

In preparation for allowing batched completions of the end_io handling,
where we can enter the flush completion with queuelist having been reused
for the batch, get rid of this BUG_ON().

Signed-off-by: Jens Axboe <axboe@kernel.dk>