blockjob: Fix coroutine thread after AioContext change

blockjob: Fix coroutine thread after AioContext change

Commit 463e0be10 ('blockjob: add AioContext attached callback') tried to
make block jobs robust against AioContext changes of their main node,
but it never made sure that the job coroutine actually runs in the new
thread.

Instead of waking up the job coroutine in whatever thread it ran before,
let's always pass the AioContext where it should be running now.

Signed-off-by: Kevin Wolf <[email protected]>