btrfs: switch update_size to bool in btrfs_block_rsv_migrate and btrfs_rsv_add_bytes

Using true and false here is closer to the expected semantic than using
0 and 1.  No functional change.

Signed-off-by: Lu Fengqi <lufq.fnst@cn.fujitsu.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>