common/rc: don't rely on /proc/partitions for device size

Non-partitionable devices, like zoned block devices, aren't showing
up in in /proc/partitions and therefore we cannot rely on it to get
a device's size.

Use blockdev --getsz to get the block device size.

Cc: Naohiro Aota <naohiro.aota@wdc.com>
Signed-off-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Reviewed-by: Eryu Guan <guaneryu@gmail.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>

common/rc

@@ -3777,7 +3777,7 @@ _get_available_space()
 # return device size in kb
 _get_device_size()
 {
-	grep -w `_short_dev $1` /proc/partitions | awk '{print $3}'
+	echo $(($(blockdev --getsz $1) >> 1))
 }
 
 # Make sure we actually have dmesg checking set up.