xfs: fix how we deal with new intents during recovery

This second series of log fixes dates back to an earlier discussion that
Dave and I had about the weird way that log recovery works w.r.t. intent
items.  The current code juggles nested transactions so that it can
siphon off new deferred items for later; this we replace with a new
dfops freezer that captures the log reservation type and remaining block
reservation so that we finish the new deferred items with the same
transaction context as we would have had the system not gone down.

v2: rework the defer capture api per hch suggestions