block: factor out a alloc_io_context helper

Factor out a helper that just allocate an I/O context.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20211126115817.2087431-11-hch@lst.de


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