common/rc: fix device size parser

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>