btrfs: switch btrfs_block_rsv::full to bool

Use simple bool type for the block reserve full status, there's short to
save space as there used to be int but there's no reason for that.

Reviewed-by: Anand Jain <anand.jain@oracle.com>
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: David Sterba <dsterba@suse.com>