Btrfs: allow for selecting only completely empty chunks

Btrfs: allow for selecting only completely empty chunks

Enhance balance usage filter by making it possible to balance out only
completely empty chunks.  Today, usage filter properly acts on values
from 1 to 99 inclusive, usage=100 selects all chunks, and usage=0
selects no chunks.  This commit changes the usage=0 case: the new
meaning is to restripe only completely empty chunks and nothing else.

Suggested-by: David Sterba <[email protected]>
Signed-off-by: Ilya Dryomov <[email protected]>
Signed-off-by: Josef Bacik <[email protected]>