block: remove get_io_context_active

Fold it into it's only caller, and remove a lof of the debug checks
that are not needed.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20211126115817.2087431-10-hch@lst.de


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