xfs: consolidate posteof and cowblocks cleanup

Currently, we treat the garbage collection of post-EOF preallocations
and copy-on-write preallocations as totally separate tasks -- different
incore inode tags, different workqueues, etc.  This is wasteful of radix
tree tags and workqueue resources since we effectively have parallel
code paths to do the same thing.

Therefore, consolidate both functions under one radix tree bit and one
workqueue function that scans an inode for both things at the same time.
At the end of the series we make the scanning per-AG instead of per-fs
so that the scanning can run in parallel.