This adds another disk accounting counter to track usage per inode
number (any snapshot ID).

This will be used for a couple things:

- It'll give us a way to tell the user how much space a given file ista
  consuming in all snapshots; i.e. how much extra space it's consuming
  due to snapshot versioning.

- It counts number of extents and total size of extents (both in btree
  keyspace sectors and actual disk usage), meaning it gives us average
  extent size: that is, it'll let us cheaply find fragmented files that
  should be defragmented.
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>