Under theoretical race condition, PHC may be destroyed while PHC
timestamp is retrieved. This will cause the PHC device memory to
be freed while being used in another thread.
To prevent such a race, PHC destroy flow will disable the PHC state
under PHC lock, (these lock and state variables are used on every
PHC timestamp retrieval).
Approved by: cperciva
MFC after: 2 weeks
Sponsored by: Amazon, Inc.