sd-device: rename devpath_from_devnum() -> devname_from_devnum()

In sd-device, `devpath` is a kind of syspath without '/sys' prefix, e.g.
/devices/pci0000:00/0000:00:1c.4/0000:3c:00.0/nvme/nvme0/nvme0n1,
and `devname` is a path to the device node, e.g. /dev/nvme0n1.

Let's use the consistent name for the helper function.

src/sysupdate/sysupdate-resource.c

@@ -608,7 +608,7 @@ int resource_resolve_path(
                 return log_error_errno(SYNTHETIC_ERRNO(EINVAL),
                                        "File system is not placed on a partition block device, cannot determine whole block device backing root file system.");
 
-        r = devpath_from_devnum(S_IFBLK, d, &p);
+        r = devname_from_devnum(S_IFBLK, d, &p);
         if (r < 0)
                 return r;