udev: use block_device_is_whole_disk()

No functional changes, just refactoring.

Note, this also makes synthesize_change() propagate the error from
synthesize_change_one(). However, the caller of synthesize_change()
ignores the failure anyway, hence the change does not take any effect.

src/shared/blockdev-util.h

@@ -14,6 +14,7 @@
 #define xsprintf_sys_block_path(buf, suffix, devno)                     \
         xsprintf(buf, "/sys/dev/block/%u:%u%s", major(devno), minor(devno), strempty(suffix))
 
+int block_device_is_whole_disk(sd_device *dev);
 int block_device_get_whole_disk(sd_device *dev, sd_device **ret);
 
 int block_get_whole_disk(dev_t d, dev_t *ret);