Discard get_sync_pos. We should be using get_resync_start.

Discard get_sync_pos.  We should be using get_resync_start.

"sync_complete" just tracks the current resync/recover/check/whatever pass.
"resync_start" tracks which parts of the array are known to be in-sync
(modulo active writes).  So it is what we need to use to update the metadata.
Also we cannot call it when the array has stopped, as the value is no longer
available then.  We must call it when the resync completes.
Possibly also call it preiodically if the array is quiescent.