Work around broken BIOS memory reporting

Andrzej has a machine with 32GB of RAM, but only 16GB is reported by the
smbios.memory.enabled.  Thus, use the greater of hw.realmem and the smbios
value.

Reported by:	Andrzej Tobola <ato of iem pw edu pl>

usr.sbin/pc-sysinstall/backend-query/sys-mem.sh

@@ -26,7 +26,14 @@
 # $FreeBSD$
 
 if smbios_mem=$(kenv -q smbios.memory.enabled); then
-	expr $smbios_mem / 1024
+	smbios_mem=$(expr $smbios_mem / 1024)
 else
-	expr $(sysctl -n hw.realmem) / 1048576
+	smbios_mem=0
+fi
+realmem=$(expr $(sysctl -n hw.realmem) / 1048576)
+
+if [ $smbios_mem -gt $realmem ]; then
+	echo $smbios_mem
+else
+	echo $realmem
 fi