btrfs: relocation: use more natural types for tree_block bitfields

We don't need to use bitfields for tree_block::level and
tree_block::key_ready, there's enough padding in the structure for
proper types.

Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>