[Libguestfs] [PATCH libnbd v3] lib: Atomically update h->state when leaving the locked region.

v1 was here: https://www.redhat.com/archives/libguestfs/2019-June/thread.html#00055 v2 was here: https://www.redhat.com/archives/libguestfs/2019-June/thread.html#00067 v3: - Fix atomicly -> atomically in commit message. - Fix a comment. - Fix TOCTTOU: There is now an inline function generated called <name>_is_permitted_state, and this is called twice, first outside the lock for the quick rejection check, and again inside the lock in case the state changed. I think an improvement would be to _drop_ the first check (outside the lock). This is an error check, so not performance critical since we are definitely going to acquire the lock on the hot path. Rich.