blockdev: split out actual DM sysfs code of get_block_device_harder() into function of its own

That way we can use it in code that already acquired a dev_t from some
source.

src/basic/blockdev-util.h

@@ -19,6 +19,7 @@
         xsprintf(buf, "/sys/dev/block/%u:%u%s", major(devno), minor(devno), strempty(suffix))
 
 int block_get_whole_disk(dev_t d, dev_t *ret);
+int block_get_originating(dev_t d, dev_t *ret);
 
 int get_block_device(const char *path, dev_t *dev);