block: return the io_context from create_task_io_context

Grab a reference to the newly allocated or existing io_context in
create_task_io_context and return it.  This simplifies the callers and
removes the need for double lookups.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20211126115817.2087431-13-hch@lst.de


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