reactor: fix flush_pending_aio() return value
reactor: fix flush_pending_aio() return value

The return value ought to indicate whether any work was done in this cycle.

dreamsxin/seastar,sjperkins/seastar,avikivity/seastar,sjperkins/seastar,kjniemi/seastar,cloudius-systems/seastar,syuu1228/seastar,syuu1228/seastar,syuu1228/seastar,dreamsxin/seastar,sjperkins/seastar,scylladb/seastar,scylladb/seastar,dreamsxin/seastar,kjniemi/seastar,cloudius-systems/seastar,avikivity/seastar,avikivity/seastar,cloudius-systems/seastar,kjniemi/seastar,scylladb/seastar