fork: move copy_io to block/blk-ioc.c

Move the copying of the I/O context to the block layer as that is where
we can use the proper low-level interfaces.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20211126115817.2087431-3-hch@lst.de


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