- Make mp_maxid reflect the same meaning as it does on other architectures.

It is one past the last valid cpuid.  This relied on a different bug in
   UMA to work properly.

Reported/Tested by:	phk
Approved by:		rwatson

sys/amd64/amd64/mp_machdep.c

@@ -179,8 +179,8 @@ cpu_add(u_int apic_id, char boot_cpu)
 		cpu_info[apic_id].cpu_bsp = 1;
 	}
 	mp_ncpus++;
-	if (apic_id > mp_maxid)
-		mp_maxid = apic_id;
+	if (apic_id >= mp_maxid)
+		mp_maxid = apic_id + 1;
 	if (bootverbose)
 		printf("SMP: Added CPU %d (%s)\n", apic_id, boot_cpu ? "BSP" :
 		    "AP");