Fix the resource.fs.zfs.statechange message

It had a number of problems:

• It was only being emitted on a transition to the HEALTHY state. That made it impossible for zfsd to take actions based on drives getting sicker.
• It compared the new state to vdev_prevstate, which is the state that the vdev had the last time it was opened. That doesn't make sense, because a vdev can change state multiple times without being reopened.
• vdev_set_state contains logic that will change the device's new state based on various conditions. However, the statechange event was being posted _before_ that logic took effect. Now it's being posted after.

Sponsored by: Spectra Logic Corp