common/rc: generalize _get_filesize()

There are some testcases use below two kind of commands to get file
size, generalize the second way as global function _get_filesize()
to simply codes.

1. ls -l $1 | $AWK_PROG '{print $5}'
2. stat -c %s $1

Signed-off-by: Chao Yu <yuchao0@huawei.com>
Reviewed-by: Eryu Guan <guaneryu@gmail.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>

common/rc

@@ -165,6 +165,11 @@ if [ ! -z "$REPORT_LIST" ]; then
 	_assert_report_list
 fi
 
+_get_filesize()
+{
+    stat -c %s "$1"
+}
+
 _mount()
 {
     $MOUNT_PROG `_mount_ops_filter $*`