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.c

@@ -20,7 +20,7 @@
 #include "missing_magic.h"
 #include "parse-util.h"
 
-static int block_device_is_whole_disk(sd_device *dev) {
+int block_device_is_whole_disk(sd_device *dev) {
         const char *s;
         int r;