btrfs: switch btrfs_block_rsv::failfast to bool

Use simple bool type for the block reserve failfast 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>