Btrfs: compression: separate heuristic/compression workspaces

Compression heuristic itself is not a compression type, as current
infrastructure provides workspaces for several compression types, it's
difficult to just add heuristic workspace.

Just refactor the code to support compression/heuristic workspaces with
maximum code sharing and minimum changes in it.

Signed-off-by: Timofey Titovets <nefelim4ag@gmail.com>
Reviewed-by: David Sterba <dsterba@suse.com>
[ coding style fixes ]
Signed-off-by: David Sterba <dsterba@suse.com>