Fix buffer overrun in readdisklabel(9) (and writedisklabel(9))
that causes unexpected panic during installation and
DIAGNOSTIC pool assertions.
Also fix bp->b_flags in writedisklabel(9) error path.

The problem was reported from Y.Sugahara during XM6i development,
and this fix is confirmed on both X68030 (by me) and XM6i (by Sugahara).

XXX: broken dkbad support (which makes struct cpu_disklabel larger
XXX: than 512 bytes) should be removed...

(tsutsui)