aio: Fix more synchronization issues in aio_biowakeup.

[jhb]

Summary

• Use atomic_store to set job->error. atomic_set does an or operation, not assignment.

• Use refcount_* to manage job->nbio.
  
  This ensures proper memory barriers are present so that the last bio won't see a possibly stale value of job->error.

• Don't re-read job->error after reading it via atomic_load.

Reported by: markj (1)

[mjg]

hrmpf

so turns out atomic_set *is* like atomic_store on linux....

mumbling ensues

that is just a side note though

another one is that the volatile u_int stuff for refcounts needs to die, but again not a blocker