btrfs: print block group super and delalloc bytes when dumping space info

When dumping a space info's block groups, also print the number of bytes
used for super blocks and delalloc. This is often useful for debugging
-ENOSPC problems.

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>