xfs: health evaluations for repair

Lay the ground work for tying the online repair functions into the new
health reporting functions.  First, repair functions need to be able to
proivde a revalidation function that will check everything that the
repair function might have rebuilt.  This will be necessary for the free
space and inode btree repair functions, which rebuild both btrees at
once.

Second, we must be careful that the health reporting query code doesn't
get in the way of post-repair revalidation of all rebuilt metadata
structures.