Use atomic load and stores to ensure that the compiler doesn't
optimize away these loops. Change boolean to int to match what atomic
API supplies. Remove wmb() since the atomic_store_rel() on status.done
ensure the prior writes to status. It also fixes the fact that there
wasn't a rmb() before reading done. This should also be more efficient
since wmb() is fairly heavy weight.

Sponsored by: Netflix
Reviewed by: kib@, jim harris
Differential Revision: https://reviews.freebsd.org/D14053