common/rc: fix device size parser

If you have multiple devices that share the same regex (eg dm-1, dm10),
then _get_device_size() can return "$size\n$size" which causes the
following error for btrfs/011.

    QA output created by 011
    ./common/rc: line 3084: [: too many arguments

So, fix this by making grep check against whole word.

Signed-off-by: Dennis Zhou <dennis@kernel.org>
Reviewed-by: Eryu Guan <guaneryu@gmail.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>

common/rc

@@ -3583,7 +3583,7 @@ _get_available_space()
 # return device size in kb
 _get_device_size()
 {
-	grep `_short_dev $1` /proc/partitions | awk '{print $3}'
+	grep -w `_short_dev $1` /proc/partitions | awk '{print $3}'
 }
 
 # Make sure we actually have dmesg checking set up.