block: Use hlist_entry() for io_context.cic_list.first

list_entry() and hlist_entry() are both simply aliases for
container_of(), but since io_context.cic_list.first is an hlist_node one
should at least use the correct alias.

Signed-off-by: Paul Bolle <pebolle@tiscali.nl>
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>