btrfs: switch btrfs_backref_cache::is_reloc to bool

The btrfs_backref_cache::is_reloc is an indicator variable and should
use a bool type.

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