Fixed allocation of unit numbers. Allocate the amount of space actually

required (rounded up a little) instead of twice the previous amount (or
a fixed amount for the first allocation).

The bug caused memory corruption when a new unit number for a devclass
was more than about twice the previous maximum one (or more than 3 for
the first one), so it corrupted memory (which happened to be the atkbdc
port resource list) in the reporter's configuration with sio unit
numbers { 0, 25, 1, 2, ... }.

Reviewed by:	dfr
Reported by:	Leonid Lukiyanets <stalwar78@hotmail.com>

sys/kern/subr_bus.c

@@ -376,8 +376,7 @@ devclass_alloc_unit(devclass_t dc, int *unitp)
 	device_t *newlist;
 	int newsize;
 
-	newsize = (dc->maxunit ? 2 * dc->maxunit
-		   : MINALLOCSIZE / sizeof(device_t));
+	newsize = roundup((unit + 1), MINALLOCSIZE / sizeof(device_t));
 	newlist = malloc(sizeof(device_t) * newsize, M_BUS, M_NOWAIT);
 	if (!newlist)
 	    return ENOMEM;